Advice on AR builds I'm thinking of

[TruthTellers]

I've decided to get an AR and I have three builds in mind, but I'd like to know if a build is the solution for them or if buying an assembled AR is the way to go.

Here's the build list:

1. Long range AR with 20 or 24 inch heavy barrel with optimal twist for 75+ grain bullets. Possibly could be .224 Valkyrie if that cartridge becomes more popular. Really wanting this to be accurate out to 500 yards.

This build does not have a weight requirement. If it weights 9 or 10 pounds, so be it.

2. Lightweight 16 inch rifle in .223/5.56 with a lightweight barrel or a pistol in 7.62x39 or .300 BLK.

I want this to be under 6 pounds and accurate out to 300 yards. The .223 I figure with the 16 inch barrel will hit hard enough that far out, but if I went with a pistol the .300 or 7.62 tends to hit hard even from a shorter barrel, but idk if a 10 or 12 inch barrel will give them enough to reach out to 300. This is the build I feel a S&W M&P15 or other rifle would suffice, but just figured I'd ask if a build is a better idea.

3. 9mm AR pistol using Glock mags.

I really don't know of anyone who makes an AR that takes Glock mags, nor do I want to get a lower that has the small magwell. I'd rather have a standard AR lower and use the magazine insert to use Glock mags so that in the event the lower gets damaged, I can easily get another lower cheap to replace it.

[Bartholomew Roberts]

A 16" barrel is accurate out to 600yds, easy. The extra barrel length gives you a little extra velocity that helps with drop, and more importantly, wind; but if you can't hit at 600 with a 16", a 24" won't change that. The shorter barrel will also be more rigid.

Also, you don't need a .224 Valkyrie for 500yds. I mean, a budget M4 barrel and a 3.5x optic with 55gr blasting ammo will let you hit e-silhouettes at 500yds.

On your .30 cal AR, either 7.62x39 or .300 will work. 7.62x39 has slightly better ballistics if you go with a 16" barrel; but the straight AR magwell plays hell with feeding. .300 BLK is designed to feed in ARs from the get go, so no problem there; but in a 16" barrel it will be slower as the round is optimized for really short barrels. Both of those will reach 300yds easy; but the trajectory is a lot less flat than 5.56.

As for a short barrel, .300 will do better than 7.62x39 in 12" and under barrels; because it was designed that way. Less flash and blast and not as bad on velocity loss.

but idk if a 10 or 12 inch barrel will give them enough to reach out to 300.

I've made hits on a 10"x14" plate at 250yds with an 8" barrel shooting 220gr flatbase subsonic .300 BLK, so I think 300yds with supersonic is very doable.

[TruthTellers]

I'm not a big .300 BLK fan, but in an AR pistol I'm starting to see its capabilities. The issue is I'm already well stocked with 7.62x39 and I really don't want to have to get involved with another cartridge if I don't have to.

The big deal is the weight. If the .300 BLK/7.62 AR pistol isn't going to weigh less than a lightweight 16 inch .223/5.56 AR, then I'm not sure if I'm interested. The idea here is lightweight and portable and .223 ammo is very lightweight and portable ammo, just really not a good hunting ammo, which the .30 calibers have it beat on.

If there's a factory AR that doesn't use a polymer lower that weighs under 6 pounds, I'll probably go with that instead of a custom build.

[Bartholomew Roberts]

ARs are just so well-supported with after-market accessories and modular, that you can do almost anything you want to do. There are ARs with AK-mag friendly lowers that eliminate the straight magwell problem. You can do super-lightweight 16" rifle builds that are under 5lbs unloaded.

And the corresponding part is there is a lot of capable .223 ammo made these days.

Honestly, I think your first two categories could be blended together easily for the ranges and weights you have in mind. However, the custom super lightweight AR uses a lot of custom, high-expense parts, so it won't be cheap.

[TruthTellers]

An AR lower that uses AK mags? YES! Do they actually work?

[Mobuck]

My thoughts:
A "long range" AR with 20-24" barrel in VAlkyrie would be a decent option. Ammo is available and not much more expensive than the high end .223.
An AR pistol in a rifle caliber is a "niche use" gun. Obnoxious to shoot and sometimes tricky to find the right combination of parts to assure function.
A pistol caliber AR pistol is "neither fish nor fowl" , unwieldy and worthless for anything except a range toy.
At least a lightweight carbine offers something you can take to the field.

[marine6680]

For 7.62x39 ARs... I hear issues with feeding from standard AR lowers... I hear issues with the AK mag lowers as well. You are shoehorning in either a cartridge into a less than optimum magazine layout, or shoehorning in a magazine type not designed for the layout of an AR... Not a great combination either way.

Building a 16in AR in the 6lb range isn't too hard. Use a lightweight barrel profile, the enhanced light weight barrels are a nice compromise of weight and strength. Then get a handguard that isn't too heavy, a 13in free float from BCM and some others are around the 10oz mark, can't get much lower without getting into some expensive or odd designs. Use a minimalist stock and you are there.

Getting much lighter starts getting expensive.

For a 500yd rifle, you don't need anything longer than 18in really.

A 9mm AR is fun, but not strictly useful in a practical sense. The idea makes more sense in a small light sub gun than in an AR.

[Bartholomew Roberts]

An AR lower that uses AK mags? YES! Do they actually work?

I haven't used any so I couldn't say; but there are several manufacturers making one.

[JeepHammer]

All over the place, so take it one at a time,

1. Long range AR with 20 or 24 inch heavy barrel with optimal twist for 75+ grain bullets. Possibly could be .224 Valkyrie if that cartridge becomes more popular. Really wanting this to be accurate out to 500 yards.

This build does not have a weight requirement. If it weights 9 or 10 pounds, so be it.

Your 'Optimum' money spent would be to buy a 'Varmint' version of AR-15, if that's the semi-auto you want.
Keep in mind, a 500-600 yard 'Varmint' bolt rifle will be less expensive, and you don't shoot rapid fire at those ranges.

I will respectfully disagree with the above, 'Varmint' barrels are usually heavier & longer than 'Traditional' length AR barrels, and for a reason...
Longer barrels produce, and maintain velocity longer than shorter barrels.
Heavier barrels are stiffer, therefore flex/cycle/deflect less with temprature or ammo changes.

I would insist on a full floated barrel in any longer range rifle, AR format included.
If the barrel needs preloaded to shoot accurately (see BAR issues), then the barrel simply isn't stiff enough to support it's own weight under fire cycling.

This will give you a lower end, lower receiver/stock assembly that will accept shorter barrel upper ends (upper receiver/barrel assemblies) that will interchange in about 15 seconds.
A 'Varmint' lower end will usually have a better trigger than the 'Carbine' ammo waster currently offered.
(A poorly done version of military trigger groups).

2. Lightweight 16 inch rifle in .223/5.56 with a lightweight barrel or a pistol in 7.62x39 or .300 BLK.

I want this to be under 6 pounds and accurate out to 300 yards. The .223 I figure with the 16 inch barrel will hit hard enough that far out, but if I went with a pistol the .300 or 7.62 tends to hit hard even from a shorter barrel, but idk if a 10 or 12 inch barrel will give them enough to reach out to 300. This is the build I feel a S&W M&P15 or other rifle would suffice, but just figured I'd ask if a build is a better idea

Again, a 'Varmint' AR lower will accept a shorter barrel upper.
I DO NOT recommend straying away from .223 here.
A carbine stock will work on both your 'Varmint' upper & carbine upper to trim things down a little.
A standard 16" carbine top end will do 300 yds without issues, just don't expect it to have the same external ballistics as a heavy, longer barrel upper, the barrel won't produce the velocity, so external ballistics will change.

As for 7.62x39R, standard AR mags won't work. You can get a few rounds to feed, but you won't get the standard 20 or 30, and 7.62x39R ammo is dirty, so it's hard on AR gas systems & the BCG.

As for .300 BO, it's lipstick on a pig, a fad round, the Paris Hilton of cartridges.
It was invented strictly because it fits in standard AR mags, and that's the only thing it has going for it.
It's an underpowered .30-30, put it in a pistol length barrel and .45 ACP outshoots it. (Energy at Target)
It's strictly a 'Tacti-Cool' fad round people buy for bragging rights, so idiots can click off 30 rounds before changing mags.

Its the crippled, retarded little brother of the 7.62x39R round, trajectory like a rainbow, usually less accurate than the 7.62x39R, and I won't even hunt with .300 BO shooters anymore since they wound more than they kill, always spraying after a running hog or coyote where a .223 with correct bullet & shot placement works 99% of the time.
The sooner it goes the way of the .45 GAP the better...

Sub-sonic, suppressed in AR pistol format it's worthless, like Paris Hilton at 60.
Strictly a 'Tacti-Cool' round used for bragging rights by guys with espresso cup holders mounted on 'Tac Rails', right next to the selfie stick, aircraft landing lights, rear view mirror, the third knife, etc.

3. 9mm using Glock mags

The 9mm cartridge has ZERO earthly use in a rifle, the only exemption is some sub-guns.
It's underpowered, it's slow, it's light, it's easy to suppress, it feeds flawlessly,
It doesn't over penetrate when some anti-terrorism or SWAT units use it, otherwise it's about worthless outside of a 5" pistol barrel.

Our SF (Special Forces) MUCH perfer the 100 year old .45 ACP round when it comes to knocking bad guys down or chewing up stuff!
230 grains, cyclic rates off the charts, controllable, easily suppressed, compact package (if you don't have tiny hands)

Personally, I have ZERO use for a sub-guns, and rarely carry a handgun.
The cheap cops in town carry Kel-Tec carbines that accept their Glock or Beretta mags.
They are fairly inexpensive, fold up to half length, come in a viriety of calibers and accept a viriety of mags.
It's not an AR, but sometimes price is a factor, and sub-gun semi-autos are nearly useless, especially in 9mm.

Just my opinion, take it for what you paid for it.

[JeepHammer]

As for AR lowers that accept AK mags...
I built some for military contractors headed to the sandbox.
Things I found out real fast,

Use a BOLT designed for 7.62x39R, this will save a LOT of failures down the line.
Most are hogged out .223 bolts and they just don't live long term.
There are a couple makers that increase the bolt face/locking lug size.
They work in standard AR bolt carriers, but you need a matching chamber nut.

The 'Standard' AR upper receiver won't allow the AK mag to lock into place, the AK mag is too wide on top.
This is solved by using a thicker 'Target' upper receiver and trimming it so the mag fits,
OR, there are proprietary uppers specifically for this conversion.
Personally, the guys seem to like the 'Target' upper with raised flat top rail rather than a rehashed military spec upper.

They say it puts sights/optics (almost always red dots) up where you don't have to cheek weld a .30 cal machine gun without having a wobbly spacer between weapon & optic.

Use a MID LENGTH (Carbine) gas system.
The 7.62x39R round doesn't produce enough pressure pulse for the 18" gas tube system, it's more of an ammo/mag thing, see below.
This is zero concern if it's a short stroke piston system.

Cut the AK mag springs to reduce pressure against the bolt.
(AK 30 round mags are WAY over sprung anyway, anyone that thumb loads them knows this!)
Friction from the spring, and that awful coating on most 7.62x39R ammo produces too much drag on the bolt carrier. Add a little sand in the lube and it's jam time on steroids.
Short gas system will overcome this about 80% of the time, but some AK 30 round mags/ammo just won't feed with full pressure springs.

Curved mags need reduced spring pressure, but interestingly enough, the older wind up drums work fine right out of the box.
If you want to click off surplus ammo, a drum is the way to go anyway!

The guys I built them for liked them as a light Squad Automatic Weapon (SAW) while using 75 or 100 round drums.
90 round AR drums have a service life, and they are hateful expensive, hate sand, fail regularly.
75 round AK drums are (relatively) cheap, grow on trees in the sandbox, and don't care about sand, are as reliable as about anything else Soviet era...

Keep in mind that contractors don't sustain battle, they lay down fire and scoot out as soon as possible, the drum fed AR-47s are MUCH lighter weight than the current SAWs, and in select fire trim do a pretty good job according to them.
AR barrels are about twice as accurate as AK barrels, and there is a substantial weight savings with AR.

PMC guys often fight actual battles, normally like whatever the military is using, I don't get that work...

[TruthTellers]

JeepHammer:
1. True, I won't be shooting rapid fire at those ranges, but I might at 200 or 300 yards and given that the barrel is longer, heavier, the accuracy and effectiveness will be better than with the lightweight AR/pistol.

Bolt would be cheaper, but the idea here is sticking to the AR platform. If you know of any bolt guns in .223/5.56 that use AR mags and can reach out to 600 yards as accurately as my custom AR with the heavy barrel, lemme know. As of right now, the only bolt gun I know of that does that is the Mossberg, but I don't know how accurate it is out to 600 yards.

BTW, the 600 yard mark is where I believe the power of .223/5.56 sputters out and becomes nigh worthless past that.

2. I'm not a fan of the .300 BLK, I agree that the 7.62x39 is superior and more cost effective. I just don't want to exclude what may be best for me. The idea here is a light weight AR that is very portable and a 0-300 yard gun. The reason I included the .30 cals there was because even in a pistol size AR, they still have the ballistics to be effective. The 55 and 62 grain bullets for .22 caliber out of a 16 inch barrel aren't that impressive.

IDK, I'm thinking that maybe I should consider the Kel Tec SU16 for this light weight .223 rifle. They use AR mags, they're decently accurate out to 300 yards, they're light, and they can fold. Oh, and they use gas pistons.

3. Goal with the 10 inch 9mm AR pistol is greater range and effectiveness over a 5 inch pistol and for suppressor use. I'm listing 9mm for now, but I may change this to 10mm. I reload and can make 10mm subsonic with a 200 grain bullet and suppress it pretty well. Thing is, I'm waiting to see where 10mm goes in the next few years and if this popularity streak it's on continues.

As for the AKAR... I'll have to think about that. I'm really just looking for a backup 7.62x39 rifle and have been considering the Ruger American for that role, but I hate that it doesn't use AK mags. Even if it just used 10 round AK mags I'd be happy. The reason I brought up AK AR lowers is part of the #2 build for the pistol.

[Bartholomew Roberts]

Originally Posted by JeepHammer

It's an underpowered .30-30, put it in a pistol length barrel and .45 ACP outshoots it. (Energy at Target)

My 9" barrel is launching a 110gr VMAX at 2200fps for about 1,150 ft/lbs of muzzle energy. Which .45 ACP did you have in mind that outshoots that!

[BWM]

Why not go buy a Varmint AR that will have a warranty I have an Windham VEX SS 20in fluted that shoots 1/2moa 100 yards 1-8 twist & Arma Lite M-15 Target rifle AR 223 20in match grade barrel that also shoots 1/2moa at 100 yards 1-8 twist You can not gone wrong this way GOOD LUCK

[mellow_c]

I'm just going to chime in on the "long range" build.

I've had great experiences with Midstate Firearms. I would recommend their 18" 416r stainless and fluted barreled upper with a 1:8 twist in .223 wylde. I've managed consistent 5/8th inch 5 shot groups from an identical upper with cheap 55 grain fmj brass cased plinking ammo. I think the 18" medium profile barrel is ideal for accuracy and a little extra velocity while still being light enough for use beyond the bench.

https://www.midstatefirearms.com/18-...-18upperss.htm

If you really want to just set up and shoot groups and are not worried about practical portability, their 24" heavy barrel would be a great option.

https://www.midstatefirearms.com/24-...-24ssupper.htm

I like their 15" gen 3 slim rail. It comes with either of those uppers at no additional cost. I think it looks good, and it has rails up front if you want to mount a bipod or flashlight or whatever. It's also a very solid and sturdy rail... It's not light weight exactly but since that's not what your after I think it would be a good option.

For the money these are really good uppers. Great quality and accuracy. They have everything ypu could ask for in regards to materials, accuracy, and general quality. I cant imagine that spending more money on an upper would give you any added performance. Read the reviews on their web site for these uppers. From there you can choose your own BCG, optic, and lower set up.

For a lower build I would run an H buffer with an adjustable carbine buffer tube and stock for the 18" upper.
Or for the 24" I might go with a rifle buffer system and fixed stock.

For a trigger I would recommend the Hiperfire 24 E for either rifle. You can put the more powerful springs in for a lighter trigger pull and more powerful hammer drop. Just do a search to check them out and to see what I'm talking about.

That trigger kit is my all time favorite and they are available from Joe Bob outfitters for only like $130 or something right now. They used to be over $200.

It wouldn't take much beyond a decent BCG, good trigger and awesome barrel to get yourself set up with an extremely accurate and reliable rifle if you build it yourself.

I'd love to give the .224 valkyrie a try, but the ammo is not cheap and for that reason alone I would stick with .223. Also the .224 valkyrie needs its own magazines.

I think an 18" or 24" setup from Midstate firearms would give you everything you want out of a 500 yard rifle. They also have the same type of barreled uppers with a 20" barrel if you wanted something between the 18" and 24".

[JeepHammer]

You can't tell me ANY subsonic round flies at 2,200 fps. Well over the 'Subsonic' range by anyone's standards.
Be butt hurt if you want, I'm entitled to my opinion just like you are in favor of .300 BO.

----

An AR-15 Varmint rifle, adjustable carbine stock or not, your choice.
Then a 16" light weight barrel top end to fit on that 'Varmint' lower end.
You can only shoot one rifle at a time, so swapping tops makes sense.
You have a 'Match' or 'Varmint' trigger.
One lower means one set of magazines, one match trigger, one high quality stock.

Carbine length barrels come in REAL light weight types.
The military used to call them 'Patrol Rifles' when they were extra light weight.
Thinner barrels often shed heat better than heavier barrels.

Medium weight short (16") barrels means support for a can if you want that particular accessory.
Even with a short, light barrel accuracy goes up with that match trigger and a stock that fits.
Adjustable stocks mean they fit even with heavy coats, load bearing vests, body armor, etc.
A rifle that fits is faster on target, accuracy depends on the shooter, all else being equal.

A pistol can't have a shoulder stock and the lower must be registered as a pistol.
For these reasons, I believe a second lower is needed for under 16" barrel.
Under 16" barrel on a rifle is an SBR (short barrel rifle) rifle registered lower makes it a class 3 weapon, need background check and BATFE approval before it's legal.

By the same token, a pistol registered lower can't have a shoulder stock or it's an SBR no matter if the lower is registered as a pistol.

There are some 'Gray' loophole devices out there, the pistol 'Arm Brace' is one, but if it hits the shoulder, it's a shoulder stock and your pistol is an illegal SBR.
If you don't believe this, there are 3,300 cases pending currently about those 'Arm Braces' pending where people were observed using them as shoulder stocks...

A 16" barrel upper for burning general ammo, speed gunning, packing the rifle around...
The varmint weight/length barrel and high accuracy ammo for those longer range shots, same lower, same mags, same accessories.
Hell, all this even fits in the same case!

[TruthTellers]

There are some 'Gray' loophole devices out there, the pistol 'Arm Brace' is one, but if it hits the shoulder, it's a shoulder stock and your pistol is an illegal SBR.
If you don't believe this, there are 3,300 cases pending currently about those 'Arm Braces' pending where people were observed using them as shoulder stocks...

All of which will be void in federal courts after the ATF said shouldering a pistol with a pistol brace isn't an SBR.

Could possibly be illegal in states/localities, but I have yet to hear of a state or locality that has deemed pistols using a pistol brace and firing off the shoulder as an SBR. Now, not all states allow SBR's, but I haven't heard of any of those states making a law that says a pistol brace on the shoulder is an SBR.

[Bartholomew Roberts]

You can't tell me ANY subsonic round flies at 2,200 fps. Well over the 'Subsonic' range by anyone's standards.

Well, you didn’t specify subsonic in your epistle. You just said .300 BLK and compared it to 7.62x39, so I assumed you meant supers. However, a 220gr Sierra subsonic is 1,017fps out of the same 9” barrel - that’s 505 ft/lbs at the barrel. Compare that to 454 ft/lbs for a Buffalo Bore .45 ACP +P. And the .300 has a much better BC, so it will have more energy on target as well.

Be butt hurt if you want, I'm entitled to my opinion just like you are in favor of .300 BO.

Project much? Opinions are fine. Your basic facts are wrong here though.

The useful part about .300 BLK is the ability to go from a Krinkov to an MP5SD with just a magazine change.

If you don't believe this, there are 3,300 cases pending currently about those 'Arm Braces' pending where people were observed using them as shoulder stocks.

Perhaps then, you could give me an example of just one of those cases. A docket number maybe?

For these reasons, I believe a second lower is needed for under 16" barrel.

An AR15 built from a stripped lower or a pistol can be converted to a Title I rifle (16” or longer semi-auto) and back to a pistol legally per ATF. An AR15 that starts life as a rifle cannot be converted to a pistol. It makes no sense; but that is the ATF interpretation of the Thompson Centerfire case.

However, you cannot have a barrel less than 16” on a stocked lower at any time. So you’d either need to swap the stock when you swapped the upper or use an arm brace for both.

[marine6680]

You can find ready made rifles with free floated 18in barrels geared for accuracy pretty easily.

The extra velocity from a 20 or even 24in barrel is not going to matter very much out to 600yds.

Longer than 20in barrels, I am not a fan of in the AR, as the gas system dwell time gets mucked with. You gain a couple inches or so of wind hold with them, eh, not worth the weight and size, and potential gas system issues.

A shorter barrel also allows the barrel to be stiffer with a thinner profile than you need with a longer barrel.

Things you gain with a heavy profile barrel... Stiffness and longer sustained fire before substantial POI shift. You also gain a lot of weight out front.

My 18in barreled AR can shoot sub MOA with mk262. I have gotten .5-.75 inch 5 shot groups at 100yds on my good days. I am more of a hindrance to the accuracy than the rifle. I can ding a 12in plate at 600yds consistently when I get my wind holds correct. And on the plains with 15mph+ winds, those holds get to be about 4moa or 2ft...

If you want to push past 600yds, yeah a 20+ inch barrel may be needed, but your wind calls will need to be spot on to do any good.


300blk feeds better from an AR than any 7.62x39 setup, and without special setups, parts and careful assembly. Yeah it costs more, and the ballistics may not be quite as good as the x39... Not like the x39 has great ballistics to begin with. It was designed to be effective out to a few hundred meters, which is where most military engagements happen.

300blk suppresses well in subsonic form, with energies around the 45acp at the muzzle, and will hold on to energy better if you use it at extended ranges... Over 100yds, due to the better BC of the bullet. Though I wouldn't want to push it past 200yds, as holds to compensate for drop will start to get a bit large, you are still working with a fairly heavy and slow moving projectile after all.

It also works well in super Sonic loads when you don't need the subsonic suppressed quietness. Then it's roughly similar to x39... So the ability to switch between sub gun quiet suppression, to open terrain effectiveness with a simple mag change, is usefull... If you actually will be doing those things.

Inside a few hundred yards, a 5.56 or 223, with smart bullet selection, is very effective on medium game... Say 250lbs and under. Take that and extrapolate as you will...

So I don't see the need for 300blk for non-suppressed use. So if you never plan on suppressing, go 5.56... it's more practical for most people. But I won't fault anyone who just wants something different, cool factor is something that comes into play.


If you want a light AR and... If you don't want to deal with the fuzzy grey areas of pistol ARs with braces and SBR loopholing... Get a 16in barreled AR with a lightweight profile barrel, and be smart with the stock and handguard selection. You can get it under 6lbs pretty easy. Beyond those measures, to go lower takes more money for fancy low weight parts.


9mm ARs can be fun, and cheap for plinking at the range. There are even competition classes that allow them... They are less than a practical choice for serious use though. Other designs are lighter and smaller usually. 300blk sub Sonic is taking over some of the sub gun role in very small ARs for those who need such things.

[TruthTellers]

Here's a follow up question.

I can't use a red dot on the lightweight 300 yard AR due to an eye affliction, but I can use a scope. That means it's going to be heavier than what it could be with a red dot, so what's a very light scope that would be good for 25 to 300 yards?

[Bartholomew Roberts]

The TA33 Compact ACOG is 12.6oz with mount and gives you a very handy fixed power (3x) optic that I can still be used close in. You want to try before you buy though as ACOG use is somehwat eyesight dependent (if you are cross dominant or have a very weak off eye, it can be more difficult to get the full benefit).

It also has a ranging reticle built in that tracks OK for IDPA sized targets; but probably not suitable for varminting.

[Kvon2]

If you're still potentially interested in the long range option, I would take a look at the stag arms m6 or varminter, whatever it's call now.

I have the complete rifle, scope and bipod. It's heavy as can be but dear Barbra is it accurate.

[marine6680]

What eye affliction?

Astigmatism? There are solutions.


The fiber optic powered reflex sights work without distortion.

There are prism scopes from 1x and higher. A 1x or 2x is not bad to use at close range. A 2x or 3x can give a bit of advantage at extended ranges. Decide which you want to prioritize, close or not.

There are several makers of prism scopes, some have ocular adjustments to compensate for vision issues.

[JeepHammer]

ANYWAY.... BACK TO THE QUESTION(S) AT HAND...

There are a lot of 'What Ifs' out there.
Some states/cities have specific laws, no laws are challenged in courts like gun laws.
When they work out bump stocks, arm braces, vertical grips & pistol grips, and all the other Tacti-Cool crap in the courts, it will still be fluff in the wind.
And it still won't matter since none of it will beat learning the basics, having a sold firearm, etc.

Unless you are working toward a rack full of rifles,
I still have to recommend a solid lower, interchangable uppers.
Saves money, weight, time, makes the firearm mission specific, and allows you interchangeability.

I would be more interested in a .22LR top end (or adapter) than some cobbled together .30 cal conversion. Common, inexpensive ammo, small game getter, seriously fun to shoot, won't be a scrounge to find ammo since .22LR isn't going anywhere.
I have a seriously old upper, 1:12" twist barrel I use with a conversion kit that is dead accurate.
The conversion consists of BCG replacement & a couple of .22LR mags, been using it for decades with zero issues, fits in any .223 upper without modification, and let's me practice indoors in the winter along with putting game meat on the table, without waste.
You can't eat a pig or deer in one sitting, squirrels & rabbits are another matter!

If you wanted to, you *Could* slap a .300 AAC/BO or whatever, top end on your well done lower.
Again, I stress the cost savings of having SEVERAL lowers, and the paperwork that follows those lowers.
It's not going to take down anything the .223 top end will, but it will give you that 'Tacti-Cool' feeling.

A 7.62x39R top will let you fire a few rounds of Russian surplus ammo before it fails to feed.
Most 20 round mags will feed 6 to 9 rounds before it fails to feed.
This failure of spitting out 30 round mags is why .300 AAC/BO exists, guys that couldn't get an AR-47 to function to use AK mags.
The AR-47 makes more sense, the rounds are as, or more powerful than the .223 case versions, and 7.62x39 ammo is dirt cheap.
When done correctly (which is rare) they are somewhat reliable and more accurate than the AK-47 clones from China.

An SKS is still the most reliable, cheapest & reasonably accurate way to burn 7.62x39 ammo.
AR-47 is the long way around the block.
.300 AAC/BO is going around the block four times just to have bragging rights for .30 cal & AR format.
I've not seen a .300 AAC/BO upper AR-15 you couldn't have bought a 7.62x51/.308 Win AR-10 clone rifle and had a highly accurate, full power round.
Exactly zero issues with reaching 800 yds with .308 Win in an AR format.
No chopped off .223 case or Russian garbage round is going to come close!

Keep in mind, that AR-10 meets your AR format requirement, you can load down from .308 case...
.22-250, 7mm-08, etc, or you can load up, .300 short mags all fit in that action length.
Just an idea if you are married to an AR format.

I keep to reasonable thinking, AR-15 in .223 is a capable round & rifle.
Mags & ammo interchange capability.
If I want longer range, or more stopping power, I simply drag out the AR-10 format or a bolt rifle.
I don't rapid fire .30 cal, so no specific need for semi-auto, other than maybe large or dangerous game.

If I were defending against humans, I would stick with standard calibers, .223-.308, the amount of ammo you have often determines outcome, standard calibers allow for more ammo without issues.
Both are often very accurate, both are easily had in volume, rifles that fire these calibers grow on trees for reasonable prices.

What works on humans works on deer/pigs.
The full size .30 cal versions work on larger, more dangerous game.
I'm MUCH more interested in eating, I'm no longer in the military, so I leave the humans to military/law enforcement.
I like bacon, ham & ribs, so I have pig guns.
I like squirrels, rabbits & other rodents, some game birds, so I have a .22LR shooter.

Keep in mind also, the AR format has been stretched so it will accommodate .30-06 up to .338 Lapua Mag.!
Not that I'll ever own one, a .308 Win or .300 WSM will drop anything I'll ever hunt, and mine reaches 1,200 yds with me behind it, so I don't see the need for a .338LM in my gun safe.
Common sense and an understanding of my (declining) abilities.

I will never chop down a .223 case reducing powder volume, then stick a .30 cal round in it to slow it down even more, just to say I have one.
Other people can go that direction if they want to, doesn't make sense to me...
If I want a slow .30 cal rifle round, I'll break out the old lever action .30-30 and down load some ammo for it.
A smooth Lever gun is still cooler than just another AR clone, and the .30-30 round is more capable!
I might even break out the old single action wheel guns and make fun of the Tupperware pistol guys if I'm feeling like it!

[TruthTellers]

So I've been watching a lot of Inrange TV's "What Would Stoner Do?" series and I think I've found the build for my #2 lightweight AR. I'm liking that GWACS lower with an Aero upper that doesn't have the forward assist, and a 16 inch pencil barrel with carbon fiber handguard. Since I'm only asking this to go out to 300 yards, I think a Primary Arms 5x Prism ACSS etched scope will do.

I'm not usually into polymer lowers, but since the GWACS looks strong and is lighter than a standard lower and buffer tube and buttstock, it's worth trying. If I don't like it or don't think it will last shooting .223, I'll use it to build a dedicated .22 LR rifle or 9mm or 10mm carbine.

[ms6852]

I own a RRA with a 20" heavy barrel that gets me out to 600 easily and boring using 75 grain bullets with a BDC reticle. The fact that you want to use heavier bullets on an AR may not be conceivable as you will exceed the length requirements that the magazine can handle as heavier bullets will be longer. Using heavier bullets will render your AR a single shot firearm. For this I would recommend a bolt action with at least a 1:7 rifle twist. Just my two cents.