Trying to break the sub 5 pound barrier, need some help

[HALL,AUSTIN]

I've built a .300 blk pistol and I am at 5 pounds 3 ounces. I've used a 7.5" faxon barrel, a midwest industry 7" free floating rail, PSA upper, run of the mill semi auto BCG and a KAK shockwave "brace". The only thing I can really think of to save weight would be a lighter bolt carrier, and maybe a lighter rail. I'd really like to avoid carbon fiber, just a preference thing. Any and all advice is appreciated, looking foward to learning a thing or two.

[mellow_c]

Do you have sights on it yet?

Depending on what you want to use for sights, that is an opportunity to save some weight. Magpuls MBUS polymer sights are an obvious lightweight choice.

I personally wouldn't mess with the BCG. I feel like if you took the weight out of the BCG you would want to add it back with a heavier buffer to keep things balanced, so no point in that. I feel like lighter weight BCGs are only meant for reduced power loads for competition shooting, but I'm not an expert, just my thoughts.

I feel like you might need to consider a carbon fiber handguard if you want to save more weight.

You could also use a basic A2 pistol grip if you haven't already as those are probably some of the lightest grips out there.

[HALL,AUSTIN]

I have the magpul MBUS on there now, and a bcm gunfighter mod1. Could adjusting the gas block mitigate the affects of a lighter bcg?

[brasscollector]

Easy. You're at 5lbs 3oz and want to get under 5lbs..just pull the buffer and spring out, problem solved

[JeepHammer]

Undercut the bolt carrier.
A milspec steel bolt carrier is WAY overbuilt for that puny .300 BO round and can be 'Relieved' a bunch without failing.

Turning will be cheaper than a titanium bolt carrier, which could also be 'Relieved' for even more weight loss, but keep in mind titanium carriers are hatefully expensive and titanium is harder to machine than steel.

An alloy barrel nut if you don't already have one.
The puny .300 BO round doesn't produce enough pressure to need the added safety of a steel barrel nut, it's NEVER going to blow out the chamber since it's never going to see combat where the muzzle might get mud or ice in it, or be fired under or full of water.

Leaving the pin bosses in place, you can machine out the sides and rear block in the lower receiver.
Again, no need for thickness/strength with the .300 BO round.
A low mass (light weight) hammer will save some weight in the lower also.

[FrankenMauser]

Drop the brace.
Multiple birds with one stone.


And... Isn't the MI barrel nut steel? Something similar, but with an aluminum barrel nut, might be lighter.


What buffer tube are you using?
Some of the "pistol" tubes can be pretty thick and heavy. And some of them designed specifically for brace use have extensions that add enough weight to be noteworthy.

[mellow_c]

If you have an adjustable gas block, you should be able to run a lighter carrier. I guess i assumed you were using a normal gas block. The suggestions above sound good to me too. You should be able to get under 5 lbs no problem!

[stagpanther]

Depends how much money you want to throw at it--could probably do it by going exotic polymer/titanium/carbon components and cutting them out. I have doubts such a weapon would be "pleasant" to shoot--but it would be a nice carry.

[JeepHammer]

A 5 pound 'CARRY' pistol (7.5" barrel)?

Actually, the printed/polymer uppers/lowers might fit this SPECIFIC application.
Those too could be slotted/thinned/lightened.
*IF* the gas pulse is tuned, and the bolt is timed right, there is very little stress on any components (other than bolt lugs/chamber nut lugs) shooting .300 BO.

About the only stresses are BCG movement/recoil spring.

I lightened up full power 5.56mm rifles, dropping the steel barrel nut for aluminum, thinned out sidewalls of lower, milled out the extra aluminum in the rear of the lower receiver, used an early soda straw barrel cut down, made an aluminum gas block with barrel turned down to it's smallest diameter, turned down/milled the bolt carrier, even shaved the shank of the bolt and lost weight everywhere I could.
Stood up to about 5,000 rounds before I scrapped it, so the little .300 BO wouldn't apply nearly the stress a full power round did.

[FrankenMauser]

That's a good point, JeepHammer.
Although my own polymer lowers are from a company that I no longer recommend and won't name, one of the 16" 'carbines' that I put together with one of those lowers ended up just a hair over 5 lbs. (Clam shell hand guards, thin barrel, thin buffer tube, poly lower.)

That 16" barreled rifle was more than a pound lighter than my 7.5" pistol. (DPMS free float tube, forged lower, heavy buffer tube.)

-Speaking in past-tense because the rifle has long since been torn apart and scattered into other projects, sans that crappy barrel. But the pistol is still here. A 7.5" AR pistol is too much fun to let get away.

[JeepHammer]

I used an aluminum barrel nut, derlin tube for handguard, turned the barrel down to smallest factory diameter, and did something I'll never do again, I made an aluminum gas block.
Aluminum & short ported gunpowder gas don't play well together.

Aluminum lives longer on full length gas systems, but every aluminum gas block I take off are eroded/corroded, so I don't use them at all anymore.

I also used derlin for recoil tube, won't do that again either. No significant weight savings, and a crap load more expensive.
Derlin is pricy, but you can turn it real thin without it failing.

The biggest weight loss was turning the barrel down, replacing the barrel nut (35 ft.lb. doesn't take much of a nut) and putting the BCG on an EXTREME diet.
Only about 1 ounce comes out of the back of the lower receiver, but keep in mind you have to leave a (paper thin) section in so it's not an 'Automatic' lower, high shelf might be a little more weight loss.

I didn't slot the lower or hand guard.
That would have removed weight but I didn't want dirt issues since I actually intended to use this in the field.
Taking the sides out of the lower/mag well will reduce weight.

I ground away much of the trigger, drilled the selector, drilled shotgun pins and used aluminum pins in trigger/hammer. I don't recommend aluminum pins, they don't live long.
Drilling/grinding ('skeletonizing') steel parts removes weight fast.

I went so far as to skeletonizing the charging handle.
It was a carry handle, so the handle went. Not my best idea...

It was light, but it was so flimsy it was a pain in the butt.
You had to use both fingers to charge, side charging would bend the charging handle.
I messed with several sights, but they weren't protected, so they got bent a lot.
Light buffer & light BCG it was a pain to get the gas right, and it was particular about ammo.
I took too much off the trigger & bent/broke the trigger pin off twice. The actual trigger was removed & a straight roll pin used for the trigger (like some 'Match' triggers of the day).

I wont be doing this again, instead opting for the most reliable rifle I can build, weight be damned...

[MagnumWill]

Skeletonized lower:

http://www.primaryarms.com/ghost-fir...-lower-sklr-tg



Or conversely, be OK with an extra 3 ounces...? What’s the impetus for breaking a perceived threshold by a particular unit of measure...?