Adjustable gas block question

[stagpanther]

Are you sure the gas block is properly positioned over the port hole? The vast majority of AR barrels are over-gassed, though shooting a subsonic cartridge in an AR pistol could also be a factor.

[JeepHammer]

OK, this isn't a joke... I thought it was...

.300 Blackout, an underpowered .30-30, cut down to pistol length which means a super short gas tube,
Subsonic is another word for crippling velocity even more
And an 'Adjustable' gas block to try and turn the gas pulse into something useable without wiping out the BCG & screwing with a dozen buffers/springs in the process...

A 'Pistol' post on a rifle forum...

AND to top it all off, no idea of the supressor design, most of which will over gas the BCG with a reasonable length gas tube!
You can forget about those 'Micro' adjustable gas blocks with a pistol length gas tube... Waste of time & money, more aggravation than they are worth!

A full gas impingement PISTOL in .300 BO, sub-sonic, with a SUPRESSOR...
... And it's NOT a joke... Since a .45 ACP suppressed will have more energy at the target!

The only way it could get worse is the gas block is aluminum...

----

OK, there are high temp thread lockers, Loctite Blue/Red is NOT them.

Once you find the MINIMUM gas port opening that functions the firearm (pistol or rifle), you CAN try counting the number of turns it takes to get the screw out,
Then wrap a (fine) copper wire in the threads, reinsert in the gas block.

This will be both vibration proof, will minimize gas leakage, will keep carbon out of the threads, and will survive high temprature.
Copper works as a sacrificial metal, it will help keep the steel screw from bonding to the aluminum gas block, much like 'Anti-Seize' keeps bolts from corroding/rusting to other components.

Since copper wire comes in VERY fine strands, the correct size is usually easy to find,
You want FRICTION with the copper, not mangling the wire as you thread it back in.
If the screw refuses to thread back in, the wire is too big...

Once you find the minimum gas passage that works,
You can also 'Scratch' or 'Ding' the threads with a chisel, deforming them slightly.
I suggest you only do the last 3-4 threads going into the block if you do this since this will slightly damage the aluminum when the screw is worked in/out.

Has the idea of a REASONABLE LENGTH gas tube crossed your mind?
The coiled 'Pig Tail' gas tube solve a lot of issues with stupid short barrels without easily eroded aluminum 'Adjustable' gas blocks...

https://www.deltateamtactical.com/Om...DE_p_4420.html

Or do a search for 'Pig tail gas tube', they will pop right up.
It's the ONLY way to control gas pulse DURATION on SBRs/Pistols using direct gas impingement.
(Pulse being defined by pressure, duration & expansion).

This *Might* restore some semblance of normality to the BCG, but I suggest an OVERSIZE charging handle head to deflect some of the blowback that's going to happen aimed right at your face!

I STRONGLY Suggest an OSS supressor for applications like this.
While expensive, I have yet to see a gas crippled SBR/Pistol they don't help a ton with the 'Issues' baffle supressors create.
They also don't screw up your gas pulse or blow back in your face like baffle supressors do.

https://osssuppressors.com

At least with an OSS supressor you can shoot full power AND subsonic rounds through the supressor without damaging it, and you won't eat as much blowback as you will with a baffle supressor.

[JeepHammer]

...when going to suppressed, would you need to adjust the gas block differently (i.e. open it more)? Or is it good to have the gas block as closed as possible for suppressors?

You are WAY off the map, there be monsters here!

Since you didn't mention a can yet, and you were shooting factory 'Subsonic' ammo...
There isn't enough information being passed along,
Will it shoot full power rounds?
You had to open up the gas block up to shoot 'Subsonic' rounds-- with or without can in place?

Having to open up the gas port using 'Subsonic' rounds WITHOUT a can in place is normal.
Subsonic will NOT produce the gas pulse full powered rounds will, and without a can there isn't any back pressure added.

There is a reason I recommend a free flowing can on these particularly difficult firearms, and I've screwed with HUNDREDS to get the firearm working with the supressor.
I'm the 'House' gunsmith for several class 3 dealers.
Everyone *Thinks* you just screw the can on and like TV, it works like FM (Freaking Magic).
They NEVER do!

You have to compensate for the crap being thrown back into the BCG by the super short gas tube which KILLS useable pulse...
You are getting still burning powder in the upper/BCG with super short gas tubes & baffle supressors! The 'Muzzle Flash' is happening INSIDE the receiver!

You have to compensate for the crap the can is going to throw back into the upper receiver, and by extension, your face...
You have to balance between full power rounds, subsonic rounds, with or without the can in place...

Trust me, it CAN drive you crazy! (Pun intended)

I set mine up like we did for the special operations guys in the military, with or without can in place, with or without subsonic ammo.
The first of the short stroke gas PISTONS & 'Pig Tail' gas tubes were developed for these guys, and they solve a TON of problems.

A military (14") barrel with full length 'Pig Tail' gas tube solved the subsonic/full power gas pulse issues with conventional baffle cans. (Direct gas impengmen)

The short stroke gas piston solved the water submersion/explosion problems AR style rifles have. (Direct gas impengmen blows the BCG/Upper apart)

The OSS style supressor solved the cycle rate increase/failure to feed issues when switching between ammo (subsonic vs full power) negating the need for specialized buffers/springs to control cycle rate & feed properly.

It's a crap shoot with a .300 BO, Subsonic, pistol length with a supressor...
Just not something the SF would use, so any information is hit & miss, personal experience from the guys that build such a hybrid, particularly if you aren't using a carbine length buffer tube...
.300 BO is an under pressure, under powered round to start with, and building it subsonic complicates the issue even more.
Always using the can will help, but it's still a crap shoot since a reputable source has yet to publish what they did to get such an odd combo working reliably, and many baffled cans aren't long lived with full power rounds...

Always using subsonic ammo & the can will UN-Complicate things some more, but there is a TON of difference between different makers subsonic rounds...
Reloading your own would give you control over the ammo issue, without being chained to one manufacturer.
(The military controls it's own ammo supply taking one vairable out of the mix)

I won't even consider an adjustable gas block that isn't steel, and doesn't have a KNOB to adjust gas passage. Just had WAY too many issues with those Allen screw mini aluminum units...
You CAN make even the military sight/gas block meter gas passage, it's easy and costs 50 cents. Most guys think a military post front sight is not 'Tacti-Cool' enough, but you can't break it off, it won't shoot loose, works with nearly all screw on rear sights, with a tap & set screw it's tuneable to the ammo you normally use... Military proven tough, and if you know how to use it, pretty damn accurate.

I actually thought this thread was a joke, sorry about that.
Hope there are some suggestions you can use without busting the budget...

[Mobuck]

Don't hold back next time JeepHammer.
FWIW
I don't feel a pistol gas system is proper for a suppressed 300 if the barrel will accommodate a carbine length. I assembled my 300's using carbine gas and they work fine. In fact, I'm going to swap the adjustable gas block to a standard and re-use the adjustable on something that needs it.
I shoot factory and handloaded ammo at various velocities w/o a lot of issues other than some pesky changes of POI.

[JeepHammer]

I don't hold back, ever.
It's an engineer thing, 'Sugar Coating' gets people killed or makes projects fail.
We are all OCD/aspergers/full on autistic, so don't expect 'Fuzzy Feelings' when hard info is involved.

It's no secret about the .300 BO & similar rounds, it was a publicity stunt that turned into a fad.
It's the underpowered little brother of as nearly worthless round, the 7.62x39R.
It was invented entirely to feed from standard AR mags, nothing more... No consideration for practical useage, just 'Tacti-Cool'.
The 'Paris Hilton' of .30 cal cartridges...

Even more worthless when fired out of a pistol length barrel using subsonic ammo.
Like Paris Hilton, Linsey Lowhan & Brittany Spears at 60... The 'Tacti-Cool' will wear off and you will be stuck with an obsolete wall hanger.

I won't hunt with .300 BO or similar shooters, they leave WAY too much game in the field wounded (if they hit it at all) and they are all AR owners, so they shoot up the country side simply because a 30 round mag fits...
It used to surprise me that my .223 put hogs down (correct bullet, shot placement) while the .300 BO guys couldn't put hogs down at the same range. Now I just won't hunt with them.

A .45 ACP, suppressed, full power rounds, does a LOT more damage in a more compact package.
Throw that into full-auto and it's a short range monster, which is why it's still used by the military to this day, 100 years after inception.
Even with a piece of crap like MAC/Ingram & Mellon thumper supressor, it would chew up the jungle like nothing else would, or can to this day.
The ONLY thing that comes close is an AA12 (12 gauge, drum fed) and you can't hide that or carry it as a second weapon very far.

If you want to mess with .300 BO, subsonic or not, that's up to you...
I'm NOT here to tell you what you *Should* buy. That's your choice.
I view it like the .45 GAP, .357 Sig and a dozen other 'Why?' calibers.

As for the OPs issues, I've run into all them before. It will get worse before it gets better.
He's already considering different gas block, different buffer/spring, ect. And it's going to be a steep learning curve, and probably quite expensive before it's over...

As for direct gas impengmen or short stroke piston,
Makes no difference to me,
-- I get paid to make either work when they get tired of screwing with it or nothing works again when the can gets screwed on...

I love supressors! They are money in my bank!
A guys spends more on the supressor than the firearm, then he can't screw it directly on and I make bank, screwing it to and the firearm doesn't work, I make bank, can't hit a damn thing & I make bank.
Since I KNOW this guy is well over $1,000 into firearm/supressor, there are no discount rates.
Supressors are strictly 'Luxury' items so if he can afford one I'm not about to discount anything!
BMW doesn't discount bills, why should I?

Grandpa's shotgun, that guy gets a break.
Someone that the 10/22 or Marlin 60 doesn't work on squirrels, that guy gets a break.
The guy with a rifle he's owned 30 years, but needs optics mounted since old eyes don't work so well, he gets a break.
Some idjit wanting a 'Tac Rail' handguard mounted in his 7th AR so he can hang the espresso cup holder, rear view mirror, selfie stick, aircraft landing lights, the 3rd knife and a dozen other things on an AR, full price plus a stupidity fee!
The idiot wanting to take grandpa's rifle out of wonderful wood and put it into a black 'Tacti-Cool' synthetic stock, full price, no stupidity fee if I get to keep the good wood...
The dummy that brings in a Jennings, Davis, Hi-Point in for a threaded barrel & trigger job, full price plus double stupidity fee...

It's a common sense thing to me.
I've been in manufacture/sales since I was 13 years old...
You TELL them what they need for the application, but you SELL THEM what they 'Want'.
Full price, No warranty, No returns. (Stupidity tax)

No ignorance tax. Ignorance simply means you don't have an education in the subject at hand... Information to someone that admit they don't know, free for the asking. I don't want dissatisfied customers.
Stupidity is when you are told better but do it anyway.

The guy has the firearm in .300 BO, might as well help him make the best of it since it has zero resale value (too specialized), he will have to find an hour's worth of sucker to come close to breaking even, so it's his for now... He stands a much better chance of selling it if it works!

[stagpanther]

Seems Sig and SOCOM like em.
http://www.businessinsider.com/new-r...ing-mp5-2017-8

[JeepHammer]

Your source is an advertisement article for Sig...

If you believe in the Easter bunny, then run out and buy one!
You might get a .300 BO that works with a supressor in pistol length that will almost remove toes when fired from your waist...
Or you can have the non-suppressed version that shoots, wait for it, 200 yds.

Sounds like a poodle shooter to me as long as the poodle isn't more than 5 feet away and isn't moving...
Makes about as much sense as .357 Sig, .45 GAP, .25 ACP...

I agree a 9mm PDW is about worthless at 200 yards, but so is subsonic .300 BO or .223 fired from a 5" barrel!

Why not a bullpup .223 with a somewhat reasonable barrel that can support a supressor?

Typical schizophrenic military thinking!
-- Chop the barrel off to nothing, put a rifle caliber round in it, then can't understand why it does NOTHING well!
Next they will be using heavier bullets and expecting that to increase accuracy & velocity like they have done with the M16 since 1965...

[Bartholomew Roberts]

I just use a normal gas block, pistol gas assembly in a 9" barrel. Works suppressed and unsuppressed, supersonic or subsonic. It is a Remington-era AAC.

[stagpanther]

If you believe in the Easter bunny, then run out and buy one!

I've been using a 300 BLK pistol for years that I built. I know what it does and how it can shoot. Thanks for your advice anyway.

[TrueBlue711]

Whew, that was quite the rant! I was able to pull some helpful info out of it though, so thanks for that. I never heard of a pig tail gas tube. For only $15, I'll give it a try. And yes, the purpose of this gun is suppressed home defense. I was adjusting the gas block for my subsonic ammo, but you're right, I should be adjusting it using subsonic ammo WITH the can attached. I doubt I'll be shooting subsonic ammo without a can on.

[Bartholomew Roberts]

A pigtail gas tube increases the time it takes for the rifle to unlock by a tiny, tiny amount. It addresses a problem caused by too mych residual chamber pressure as the bolt tries to extract and is caused by the extra backpressure and dwell of a suppressor plus usually an eroded gas port.

It is not going to fix any problems you have with subsonic .300 ammo, surpressed or not.

[stagpanther]

I've developed a hot load for 110 Tac TX bullets that out of my 7.5" pistol are fast, accurate and very easy to shoot free-hand with minimal recoil. The only drawback is that it makes a big bang for it's size. Not for everyone, but a viable and effective weapon within it's limits IMO. Not many pistol cartridges around that can shoot real world rifle bullets.

[TrueBlue711]

Update: Just got my suppressor on Friday and took everything out to the range Sat morning. Made a huge difference adjusting the gas block with the suppressor on. Everything works like a champ now. Only problem is the bolt won't lock back after last shot fired when using subsonic ammo w/out the can attached. Not a big deal since I'll typically only use subsonic ammo with the can attached to the gun.

[JeepHammer]

Now, if a killer poodle would only freeze right in front of you...

[Sharkbite]

So MUCH bad info in this thread. Sad to see.

The 300blk is purpose built to run out of a 9” barrel. Thats the original barrel length AAC put on the guns (I have one). That gun does NOT use an adj gas block, just the proper FOR THIS CALIBER port size.

My gun runs like a sewing machine in all 4 modes (super, subs, can or no can).

Most of the advice given above is true for a 556 gun, NOT for a 300blk gun. Totally different animals. Port pressures, gas volume, powder burn rates. All are COMPLETELY different 556 vs 300blk.

Even the caliber comparisons (45acp vs 300blk) is flawed. Now, if you dont like the caliber...thats fine. Dont use it, but to throw out such misleading info is not responsible and does the forum membership an injustice.

So, AAC (who brought the caliber mainstream) specs a 9” barrel with a PISTOL length gas tube and a fixed gas block. To step outside that system can lead to issues. The only reason i can see for an adj gas block IN THIS CALIBER is to be able to turn the gas OFF. For suppressed use that will eliminate the action noise and make the gun quieter

Adj gas blocks on 556 guns make sense for OVERGASSED guns. A 300blk is very rarely overgassed.

[riffraff]

For awhile I've been meaning to look into what the deal was with adjustable gas blocks. This post just convinced me - the deal is it's something to avoid at all costs unless no other option exists

[disseminator]

So reading this thread one might get the impression that the 300 Blk is a finicky worthless round that only big fat dummies will use and is only useful for 200 yrds.

That is so wrong.

I don't have a suppressor, but my 16" pistol length gassed DI 300 Blkout will run 110g to 220 grain all day long and is MUCH superior to a 45 Auto, especially in a military application which it was developed for.

Never seen a 45 shoot 1000 meters

[stagpanther]

I've had a 300 BO pistol for years. I've loaded my own ammo and blueprinted the velocities with labradar. It's not a 2 mile sniper cartridge--but is certainly far superior to any typical pistol cartridge IMO. What sets it apart is that in the short-barreled non-stocked configuration it is easy to shoot, low recoil and keep on target. It is plenty powerful withing it's intended range. Nothing else quite like it IMO. Like or don't like; I stopped listening to the drivel long ago--I was once one of the 300BO downers myself--until I built one.

[Bartholomew Roberts]

Never seen a 45 shoot 1000 meters

That’s “only” 750m (with an Aimpoint T1 no less). And Travis Haley, who has made shots on people in actual combat at distances of over 600yds with a 16” AR. Still, at 750m, you are looking at about 39 FEET of drop on a 110gr VMAX from a 9” barrel.

Certainly doable on a known distance range with zero wind value; but you’ll have to be able to range very accurately and call wind like a champ to make hits at that range (a 2.5mph error in crosswind is about 35” of drift at that range).

[JeepHammer]

"Typical Pistol Cartridge"

Are you kidding me?

First off, a vast majority of .300 BO barrels are 16", rifle tops.
This doesn't cover a bunch of bolt rifles chambered in this strange hybird round.

Second, how are you going to wrap your hand around the grip of a 'Hand Gun' with a 35mm long round magazine in the handle?
Many have issues with .45 ACP at 23mm long?

Third, let's admit it, chopped off ARs aren't 'Pistols', they are chopped off rifles.
Balance/ergonomics out the window, awkward to say the least and WAY over complicated for a 'Pistol'.
Forget quick deployment...

It's nothing the 5.7x28 FN didn't have covered, and that round works in an actual handgun, along with the P90 PDW that's an excellent purpose built PDW, and the round interchanges into actual handguns.

Might I add that the P90 comes factory with 50 round mags (Tacti-Cool guys take notice!)

This particular round is used by military & police in 60 countries (including US) so it's battle proven & dependable.

8" barrel, 2,350 FPS, effective range 230 yds (iron sights), maximum range over a mile. Battlefield hits commonly reported at 500 yards.
It's a simple blow back design, no explosions from being submerged, virtually jam free, 900 rounds per minute in full auto, fires from closed bolt.

Keep in mind that HK tried a 4.6x30mm round in PDW, so the concept is/was viable for more than one manufacturer/country.

[Sharkbite]

I think your tag line describes your interaction perfectly

I know A LOT of guys that have an AR chambered in 300blk, VERY few have a 16”+ barrel on those guns. Pistols and SBR’s rule this space. Just about everyone i know with a pistol, has a “Brace” on it (nuff said)

The 300blk allows my to stick in a mag of 125gn supersonics and hit 7.62x39 (darn near 30-30) balistics. With a simple ammo swap, im shooting 210-220gn subsonics. No fiddiling with the guns gas system, no BCG swaps. Just change out the ammo and go.

Both types (subs and supers) run thru my silencers just fine. Supers are louder of course, but subs are giggle quiet.

As to your handgun power comparison, a 200+ grain rifle length bullet at 1000fps is NOT the same As a 45acp bullet. Ballistic coefficient is not even close. With the major improvement in expanding bullets for this calibers subsonic loading, its worlds apart from the 45acp.

Id suggest some real research on your part before misleading folks on a internet forum. At best your info is outdated and incorrct.

P.S. the gun AAC developed was for a contract let by a USSOCOM group. Sit at a bar at SHOT show and talk to the guys that helped develop that gun. Very enlightening

[JeepHammer]

The .300 BO was a 'Pretender' for the PDW NATO replacement, not SOCOM.
That NATO...

While the 'Pretender' .300 fell flat on it's face, being rehashed from the 1958 NATO contender cartridges,
The 5.7x28mm met all the 1990 NATO requirements and went into service in 1993.
Now used by 60 countries, military & police, combat proven.

In the meantime, the rehashed .300 BO has been marketed to US citizens that don't know about the NATO trials, and continues to be pushed by US military contactors, even though it fails every NATO test.
Repeating that, AAC has been trying to sell the .300BO to the military since the 1990 NATO trials, and has repeatedly FAILED against several other rounds/firearms.

It shoots in AR clones, and that's all the US civilian looks for... Which is the vast majority of the .300 AAC BO market.
They spent a ton of money on that rehashed product, why loose that money when they can market to civilians that don't know any better...
Sell it in a 'Black Gun' format, and it's sure make some sales...

In 2017 US SOCOM asked for submissions in specific caliber .300 AAC Blackout (after AAC lobbied the military for 20 odd years) no other calibers allowed to directly compete with the .300 AAC Blackout round.
If you don't understand this, it's a rigged competition since specifications were written specifically for the .300 AAC Blackout round, no other rounds will even be considered since the .300 Blackout has lost to several other rounds in ALL other previous competitions.

It's underpowered compared to the 123 year old .30-30, (published at 2,390 FPS, 150 gr bullets)
This means it can't compete with a round nosed bullet in the very first rifle round ever devloped for modern 'Smokeless' (nitro-cellulose based) rifle propellent ever in a standardized round from a major US manufacturer (Winchester 1895 rifle).

30 carbine has 2,000 FPS (7.62x33mm), 110 grain bullet.
The idea of a .30 caliber round in a shortened case isn't new...
This little 1938 designed round, 80 years old now, is neck in neck with the .300 BO in muzzle velocity.

.45 ACP, 185 grain bullet, 1,100 FPS, 230 grain bullet, 960 FPS. 5" Barrel.
I'm sure velocity beats .300 BO with 9" barrel, but it's not commonly published information.

.300 BO, no credible source publishes data for anything under 16" barrel,
.300 BO, .7.62x35mm, 16" barrel, 220 grain bullet, 1,010 FPS.
125 grain bullet at 2,215 FPS.

.45 ACP, 5" Barrel, 230 gr round, 960 FPS,
.300 BO, 16" barrel, 11" MORE BARREL,
220 grain bullet, 10 grains LESS than .45 ACP, 1,010 FPS.

Your claims don't seem to live up to published facts...
SO,
How is that for ACTUAL facts...
Just a pointed version of .30 carbine, underpowered .30-30, been a waste of time since the 1958 NATO trials, AAC had to rig a competition to get it considered for anything since it can't compete with other offerings currently in use around the world...

It doesn't matter how may coats of polish you put on a it's still an underpowered carbine round,
It was underpowered in AK-SKS, its been an under powered, under performing round in all NATO head to head competitions, now it's even more underpowered underpowered in current trim, especially in a pistol length barrel hanging off an AR clone lower.

The ONLY positive is it will fit in standard AR 30 round mags.
Personally, I don't shoot 30 round mags (because I aim) and I don't use rifle actions chopped down into 'Pistols'.
There are a TON of actual pistol frames/rounds out there, I see no point in chopping off a rifle for civilian use...

If YOU like your .300 BO, that's fine for you & I don't care what YOU like or shoot,
Just don't claim it's the 'Perfect' round, the chopped down AR the 'Perfect' format, or claim it's capable of anything it isn't.
It's simply a slow .30 rifle bullet in a carbine/pistol length case, nothing more, nothing less.

It's a fad round right now, it's been around in one format or another since 1958 NATO trials,
It's been discredited by US military & NATO, loosing every head to head competition it's been in...

[Bartholomew Roberts]

Originally Posted by JeepHammer

Repeating that, AAC has been trying to sell the .300BO to the military since the 1990 NATO trials, and has repeatedly FAILED against several other rounds/firearms.

AAC didn't even exist as a company until 1994 and development on .300 BLK didn't start until 2009. .300 Whisper as patented by J.D. Jones existed in 1990; but Jones has no connection with AAC and has never competed in any military trial.

In 2017 US SOCOM asked for submissions in specific caliber .300 AAC Blackout (after AAC lobbied the military for 20 odd years) no other calibers allowed to directly compete with the .300 AAC Blackout round.

Again, development started on .300 BLK in 2009. It wasn't introduced until 2010. Yet, you are telling me that three years after starting the company, AAC started lobbying the military to use a cartridge they wouldn't start developing until 12 years later? That sounds like a fascinating story. Why don't you tell us the details?

Originally Posted by JeepHammer

It's underpowered compared to the 123 year old .30-30, (published at 2,390 FPS, 150 gr bullets). This means it can't compete with a round nosed bullet in the very first rifle round ever devloped for modern 'Smokeless' (nitro-cellulose based) rifle propellent ever in a standardized round from a major US manufacturer (Winchester 1895 rifle).

What barrel length? And how much energy are those round nosed bullets still carrying at 200yds? Also, I believe you mean 1894.

30 carbine has 2,000 FPS (7.62x33mm), 110 grain bullet
The idea of a .30 caliber round in a shortened case isn't new...
This little 1938 designed round, 80 years old now, is neck in neck with the .300 BO in muzzle velocity.

So, with a 16” barrel, the .30 carbine shoots a 110gr bullet a full 200fps slower than my 9” barrel AAC?

.45 ACP, 185 grain bullet, 1,100 FPS, 230 grain bullet, 960 FPS. 5" Barrel. I'm sure velocity beats .300 BO with 9" barrel, but it's not commonly published information.

Typically, the only heavyweight loadings in .300 are intentionally designed to be subsonic out of a 16” barrel. As far as I know, nobody is trying to push a 180gr bullet as fast as possible in .300. The closest information I have is 1,447fps (avg) out of a 9” barrel for 168gr Fusion. 220gr Sierra Match Kings are launching at 1,017fps avg out of my 9” barrel.

.45 ACP, 5" Barrel, 230 gr round, 960 FPS,
.300 BO, 16" barrel, 11" MORE BARREL,
220 grain bullet, 10 grains LESS than .45 ACP, 1,010 FPS.

Well, as I pointed out one of those rounds is being intentionally loaded to be subsonic. That’s the point.

Which beings us full circle to the niche the .300 serves. If you need a compact firearm that can go from MP5SD quiet to intermediate caliber rifle with just a swap of the magazine, there aren’t any other choices available.

[JeepHammer]

Still beating a dead horse.
The round has been around in one form or another for 100+ years (see Mauser).
Mag in front of pistol grip so it's awkward to say the least, the AR is just the latest underpowered, awkward offering in an underpowered rifle round in a long line of failures.

YOUR specific choice of short rifle caliber is just another example of this 100+ year old trend.
Since you can't figure this out, let me explain it again...
Europe wanted a NATO round long before your specific chambering. They all sucked hard and have been dismissed in turn for reasons stated above.

It's an underpowered knock-offs of the 7.62x39R, which was a knock-off of German WWII rounds.

It was, always has been, and will continue to be a substandard battle rifle cartridge, and it will continue to be a horrible pistol cartridge.
It *Might* have a niche as a PDW cartridge, but it will be substandard for that role as long as rounds like 5.7x28 in firearms like the P90 exist.

The Europeans tried and rejected a dozen rounds very similar to, but even more potent.
NATO rejected the short rifle rounds, the US rejected the same offerings.
It doesn't matter what incarnation someone comes up with next, it's simply an inferior round on every level, and no amount of fan boys are going to change the physics of the round...

You simply can't accept facts, NATO scrapped it in 1958, US scrapped it in 1960, no amount of misconstrued BS is going to change physics.
Since you don't distinguish the entire GROUP of short case rounds and try to differentiate between all the failures as a whole, there isn't much anyone can do for you.

[stagpanther]

good grief.