AR Build: Pistol to SBR. What caliber and what barrel length?

[Olympus]

I have a spare lower that I need to turn into something. Keep in mind that my spare lower is a consecutive serial number to the lower I used for my AR rifle. Basically I'm wanting to have a nice pair of guns.

I'm thinking for my second lower, I'll build it into a pistol initially and put a Sig brace on it while I wait on a tax stamp to turn it into an SBR. There is no real purpose or intended use for the gun. It will be a fun gun, a truck gun, a SHTF grab-and-go gun. Basically a mate to my 556 rifle with the next serial number.

But I'm stumped on two parts, caliber and barrel length. My caliber choices are 556 like my other gun, 300BLK, or 7.62x39. My next problem is 7.5" or 10.5" barrel?

556 is the safe bet. Easy to get cheap ammo and my mags will interchange.

300BLK would be my first choice, but ammo is hard to find and really expensive. It doesn't lend itself to fun shooting. Reloading is an option, but even finding brass is impossible in my area.

7.62x39 would be even cheaper for ammo. Better ballistics from a shorter barrel. But you have to use those goofy looking bent mags and they won't interchange. And I also heard that 7.62x39 from a short barreled AR is not very fun to shoot due to recoil and concussion wave.

Then for barrel length I planned on using 7.5" because I wanted to be very short and easy to conceal in a bag of some kind. But people are saying 7.5" isn't long enough with 556 to be very effective and that 10.5" is necessary to get decent ballistics. Then others will say 10.5" is getting close to full rifle size and less like a "pistol/sbr". So I don't really know what to do about this either. Any advice would be helpful.

 

[crazysccrmd]

Best option is a 10-12" 6.8 spc. Way more power than a 5.56 or 300blk, better function and feeding than x39. If you want to stick with 5.56 I'd go with a 10.5", I have one and it's still very compact.

 

[C-grunt]

I'd do a 11.5 5.56 gun. More reliable than the 10.5 and barely any longer. I have friends and family with SBR ARs and they are much more handy than 16 inch rifles.

 

[dumluk]

For 5.56 I went with a 10.5" Anything shorter and you've created a flame thrower with poor terminal ballistics. In 300 you can run a 7.5, the round is engineered for short barrels. It was also engineered for shooting suppressed/subsonic and if you don't plan on suppressing its, as you stated, too expensive to plink with. You can get comparable ballistics out of those funny looking mags for a lot less.

Best this about your dilemma is that you don't actually have to choose. SBR the lower and build all the uppers you want for it. When your stamp comes back build another pistol lower and move the sig brace.

 

[MistWolf]

The most practical and economical SBR is an 11.5" 5.56. The 7.5" barrel is much more difficult to get running correctly and gives up too much in ballistics. The shortest practical SBR for the 5.56 is a 10.5".

If you'd rather have a 300 BLK, run it. If not, don't. It's really as simple as that. The 7.62x39 has a lot of problems to overcome to get it to run in an AR rifle.

Whoever told you that a firearm with a shorter barrel recoils more didn't tell yo the whole story. A shorter barrel may have more muzzle blast, but produces less velocity. Less velocity reduces recoil. Now, if by shortening the barrel it reduces the weight, that increases recoil. The two may or may not cancel each other out.

The reason many believe a shorter barrel rifle has more recoil is because it has more muzzle blast. More muzzle blast has to mean it kicks harder, right? Bit that's not so.

Chose the caliber for your SBR you want. If you want to go shorter than 10.5", get the 300 BLK. If you want the 5.56, the 10.5" is the shortest practical length with 11.5" being even better

 

[wally]

.300 blk only if you like to reload

~11" barrel if you plan on using a suppressor eventually.

I really like my 11" AR 7.62x39 SBR. Problems I had were not insurmountable. My friends Model 1 Sales 7.62x39 pistol kit worked 100% from the gitgo (mine was an MA Parts upper, because they used 1/2x28 threads and I already had a suppressor mount in this thread).

Real issue is the 30 round mags -- many need a real AK 47 magazine spring and may only be really reliable with FMJ ammo, not likely an issue in "only a fun gun". The "new" 20 round ASC mags hopefully solve the problem.

And I also heard that 7.62x39 from a short barreled AR is not very fun to shoot due to recoil and concussion wave.

In my experience its not bad and 5.56 is worse.


5.56 has the advantage of caliber and mag commonality, but you really give up a lot with the short barrels.

I'd also consider a 5.5" or 7.5" 9mm. Ammo is cheap and they are a lot of fun to shoot. Like with 7.62x39, the mags can be an issue but I've found they all seem to work eventually after some internal finish has worn off so the rounds move freely and don't bind.

 

[Collector0311]

I'm looking at a 9.5" because from what I've heard on this forum, is that 300blk is designed to burn in 9.5" of barrel.
This also puts your weapon back near standard length with the addition of a suppressor.

I'm also running a parallel 5.56 to my 300blk and have been saving all my 5.56 brass to get into making my own 300blk brass.
Good luck with your decision!

 

[EdLaver]

+1 what Wally said, 300 blk is ideal for shorter barrels, but you'll have to reload to enjoy it...unless you are rich

 

[Olympus]

300BLK is pretty much out of the running. It's too high for me to use for volume shooting.

 

[wally]

The decision is not really a major issue, as once you get your stamp you can have multiple SBR uppers Just don't have extra lowers around without rifle uppers to go with them.

 

[strambo]

I'll echo the common advice already given. Avoid 7.62x39, not reliable enough. If expensive ammo and/or reloading is a no-go then avoid 300 BLK and 6.8. That leaves 5.56, get at least a 10.5" barrel for reliability.

I bit the bullet and SBR'd a lower and went with a 10.5" 5.56. No issue so far, I really like it.

 

[bldsmith]

I am building a 10.5" pistol right now. Just put the handguard on yesterday (sorry no pics yet). But I went with NONE of the above My choice was the 458 SOCOM. Only lose 50fps over the 16" as it was really designed for a 13" barrel. Here is a shot of parts I had a few days ago. RRA 458 on top.



Much more compact than the 16. Also planning to SBR it as soon as I can afford another lower.

 

[gotigers]

I have a 10.5” 5.56 and I have zero mechanical issues. 10.5 is the shortest I would go. Many suppressor manufacturers will not warranty shorter than 10.5 and 10.5 is the shortest while retaining enough velocity with heavy OTMs to still have a useable expansion threshold.

The only negative is the loss of velocity vs an 11.5”. I wanted the shortest I could go and still have a useable expansion threshold and the keep the overall length as short as possible with a can. Mine is equivalent to an AR with a 17” barrel with the can. If I weren’t getting a can, I would probably go 11.5 or 12.5.

Regarding reliability, just get a barrel from a manufacturer with a good track record: Daniel Defense, FN, etc. They will have the correct size gas port because they take the time to test and have been doing this a long time. I have a FN 10.5”, 1:7 twist, NATO chamber, Chrome lined, H2 buffer, milspec carbine buffer spring, semi auto BCG. It shoots everything from Tula 55gr steel cased to 50gr JHP Remington, 77gr MK262, 75gr T2 TAP. It shoots those with or without the can (SWR 5.56 Specwar).

If you want shorter than 10.5, go with 300blk. You can go as short as 8” with 300blk. Then ammo availability might get spotty.

 

[Theohazard]

Also planning to SBR it as soon as I can afford another lower.

Why don't you just SBR one of the lowers you already have? Many people SBR their pistol lower, so when the stamp comes in they just switch out the buffer tube and add a stock.

 

[bldsmith]

So I can keep the pistol lower intact for ease of swapping. I want to be able to take the pistol to Tx pig hunting without having to ask permission. Also might do another short barrel build needing a lower. Good way to get another AR with fewer hoops to jump through. Or at least I can do it a little at a time.

 

[tulsamal]

I have to echo the unlisted alternative. 6.8 works really well out of shorter barrels. Fatter and heavier bullet than 5.56mm. Lots of fancy Barnes bullets for serious stuff. Great pig cartridge. I would go with something like a 10.5" 6.8.

Gregg

 

[benzy2]

I'm not up to date on what a 6.8 can or can't do from different barrel lengths so I'll leave that to those with more knowledge.

If you are looking at sticking with a standard sized BCG, the 300 blk is far happier on a pistol length than as a carbine, and it's far happier on a small pistol length than .223/556 from the same small length. Certainly 300 blk can be made to work in longer barrels and .223/556 in shorter barrels, but you hit the pressures and time frames you want with a 300blk in those short pistol barrels. In the longer length pistol barrels, it's a toss up either way.

You also have the ability to stay subsonic if you want to play with a suppressor using the 300blk.

Now, the big drawback and the one that keeps me from shooting mine much, is ammo. Reloading is fairly straight forward, but the powders for the different bullet weights are a little hard to find these days as quite a few 300blk reloaders are stocking up. It's also not the round you want to buy if you plan on not reloading in bulk. The work to convert a .223 case to a 300blk is more effort than I want to put in. I bought a bunch already converted and annealed but it's still a long day at the loading bench where .223/5.56 can be found quite easily these days and for a decent price.

So, long post short, 300blk is a better functional choice in the shorter pistol barrel lengths, 5.56 and 300blk are fairly equal in middle of the road length pistol barrels, and 5.56 is a better choice in longer pistol barrels. If you reload, 300blk is feasible and if not go with 5.56 without a doubt.

Personally, after getting bit by the 300blk bug, I wish I wouldn't have. I consider swapping barrels out for a 5.56 barrel every day as I know I'm just not going to spend the time to load 300blk in the quantities I want to shoot. On the other hand a middle of the road 5.56 barrel and 10,000 rounds of wolf/tula ammo seems more likely.

 

[dogmush]

I have a10.5 5.56 "pistol" it's reliable and fast, but I wouldn't go any shorter. The blast is impressive.

I also have a .300BLK. It's one of my favorite calibers even unsupressed. It is more expensive to shoot though.

Both are good weapons. If I could only have one, I'd go 5.56 just for cheap available ammo. But get both if you can.