12.5” AR-10 .308 SBR Thoughts?

Tonto20

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I acquired an old large frame billet lower from a company that is no longer in business Legion Firearms. And I’m considering Form-1-ing it for a fun, piss off the neighbors (when not suppressed) SBR. Anyone running one got any experience with one?

Or any advice on best places to source compatible parts for an SR25 pattern lower?
 

Aescobar17

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Loud, heavy, and not much fun. Very effective for a short barrel though. I wanted one til I ran one a bit. I lost the urge pretty quickly. Took a longer heavier can to make it bearable.
 

samson7x

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I just started a 12.5 "just for fun" build too, so I'll be following this thread.
 

garandman

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I know my 18" DPMS LR-308 was essentially field artillery.... sold it pretty quick. Too heavy. And cuz my can "shovelled" carbon and crap back into the receiver.
 

SkyScrapin

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Similar, but different. I built a 11" 308w bolt gun. It's heavy. Loud as a hell without a suppressor, and loud with a suppressor (SAS Resistor). I'd be careful which can you use because most manufacturers will not warranty that short barrel. 135g SMKs are the ticket.

If you can get over the above, I can guarantee it will be one of your most favorite dumb ideas. About as fun as it get, and damn near perfect for a blind/truck gun (wear ear pro).
 

garandman

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Similar, but different. I built a 11" 308w bolt gun. It's heavy. Loud as a hell without a suppressor, and loud with a suppressor (SAS Resistor). I'd be careful which can you use because most manufacturers will not warranty that short barrel. 135g SMKs are the ticket.

If you can get over the above, I can guarantee it will be one of your most favorite dumb ideas. About as fun as it get, and damn near perfect for a blind/truck gun (wear ear pro).
Good point. Baffle strikes / baffle erosion from getting sandblasted with unburnt powder are significant considerations.
 

Tonto20

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Interesting, I appreciate the input, I’ll be matching it to a SiCo 7.62 Specwar can. It’s a heavy bitch alone. Will definitely research the 308 barrel length rating.

Would and adjustable gas block help tame some of the blowback or is that just trade off of having a short barrel?
 

garandman

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Interesting, I appreciate the input, I’ll be matching it to a SiCo 7.62 Specwar can. It’s a heavy bitch alone. Will definitely research the 308 barrel length rating.

Would and adjustable gas block help tame some of the blowback or is that just trade off of having a short barrel?
My understanding is that most of the blowback is not coming thru the tiny gas tube, but the wide open gaping hole called the "bore." :)

There's no free lunch, here. Shooting gas guns suppressed is messy.
 
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USMC_JA

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Strongly suggest you build that .308 as a pistol if you are determined to do it. No way I'd SBR a large frame AR only to find out that it sucks. You've then wasted $200, the time involved in waiting for approval, and engraving.

Build it up as a pistol and try it out first. If you like it, then SBR to your heart's content if you want so you can run a correct stock.

Additionally, a 7.5 inch AR pistol can be a very loud, impressive asshole. Here is my version of such a thing. It never fails to make folks move a few lanes down from me or draw a crowd. With an Echo trigger, it is an absolute riot.

7094097
 

Tonto20

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Strongly suggest you build that .308 as a pistol if you are determined to do it. No way I'd SBR a large frame AR only to find out that it sucks. You've then wasted $200, the time involved in waiting for approval, and engraving.

Build it up as a pistol and try it out first. If you like it, then SBR to your heart's content if you want so you can run a correct stock.

Additionally, a 7.5 inch AR pistol can be a very loud, impressive asshole. Here is my version of such a thing. It never fails to make folks move a few lanes down from me or draw a crowd. With an Echo trigger, it is an absolute riot.

View attachment 7094097
That’s an outstanding suggestion. Guess I’ve never thought of it from those points before.
Thank you gents.
 

AK4900PA

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Almost done with mine, just need the 12.5" .308 barrel and BCG. The form 1 was approved ~6 months ago, but it got put on the backburner for a bit to fund a couple new pistols.20181028_180535.jpg
 
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Yondering

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My understanding is that most of the blowback is not coming thru the tiny gas tube, but the wide open gaping hole called the "bore." :)

There's no free lunch, here. Shooting gas guns suppressed is messy.
I've noticed you only seem to know about half the story on most of the things you're giving suggestions about here. The above is a good example, you're clearly guessing based on theory without having done it yourself or at least understanding the entire system.

To the OP - yes, tuning your gas system (via adjustable gas block or other methods) makes a big difference shooting suppressed; the main goal is to delay bolt unlocking until the correct time. That does reduce the gas back into the receiver (and your face) by a lot, and is definitely worth doing if you'll shoot suppressed very much. With that short barrel, gas management when suppressed is a big deal, more so in my experience than with longer barrels. Also, expect it to still be fairly loud suppressed.
 

towerofpower93

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I enjoy mine.

Certainly not the most logical firearm I own, but fun none the less.

As far as velocity goes, it doesn’t lose as much as some think.

Here are the loads I started settling in on, 10rds per group:

Win 748 47.2grHornady 150gr FMJBTWin LR2.700"Avg: 2518
ES: 33
SD: 11.2


Win 748 47.2grSierra 155gr TMKWin LR2.800"Avg: 2553
ES: 64
SD: 17.9

And some FGMM for comparison, 20rd box of each:

Sierra 168gr MatchkingUNKAvg: 2341
ES: 37
SD: 10.8


Sierra 175gr MatchkingUNKAvg: 2310
ES: 55
SD: 14.7

8208XBR is giving similar velocity to the W748 with lower ES/SD and I think I’ll be going that route long term with the 155TMK for distance and 150gr Hornady FMJBT for hoser stuff. Don’t have enough rounds down the pipe/past the LR to post data yet.

Adjustable gas block is a must, but I think every .308 AR should have one in the civilian world where you’re not tied to one or two rounds like the .mil
 
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garandman

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I've noticed you only seem to know about half the story on most of the things you're giving suggestions about here. The above is a good example, you're clearly guessing based on theory without having done it yourself or at least understanding the entire system.

To the OP - yes, tuning your gas system (via adjustable gas block or other methods) makes a big difference shooting suppressed; the main goal is to delay bolt unlocking until the correct time. That does reduce the gas back into the receiver (and your face) by a lot, and is definitely worth doing if you'll shoot suppressed very much. With that short barrel, gas management when suppressed is a big deal, more so in my experience than with longer barrels. Also, expect it to still be fairly loud suppressed.

Basic gas law....it takes the path of least resistance....even if you magically find this ideal time to unlock the bolt....which you have absolutely NO way of knowning if yer doing, cuz the time btwn the primer igniting and the bullet leaving the barrel is about 1/1,000th of a second. The path of least resistance is the bore. THAT is where the gas/ crap is gonna flow. I've got adjustable gas on my .223 AR. I've fooled with its settings, found the point it just barely consistently cycles. That is CLOSE to that magical ideal moment. It STILL dumps a crapload of carbon into the receiver. (I've used both Seekins and Superlative Arms type adj gas) If 25-26 grains of 223 powder does that, 40+ grains of 308 will do it more. Best bet is to run 208-220gr heavy bullets subsonic. But as was said above, better to just get / build a 300BLK, than a 308.

Yer "magic moment" theory violates basic physics. There's no free lunch, here. Suppressors create back pressure. ALL of them, on every gas system. That backpressure exists WHILE the bullet is in the can, after the bullet has left the barrel. At that moment, the path of least resistance IS ** the bore.*** You suppress a gas gun, yer gonna get a ton of crap dumped in your receiver. Suppressor MANUFACTURERS WAY smarter than both of us have confirmed this. (Its still fun to do, anyway.) An adjustable gas block helps a little, but not much. Best avialable solution is a piston system....which STILL has the bore / chamber as the point of least resistance. I run those too, in 300 BLK, subsonic, and 223 supersonic. They gets WAY more crap in the receiver that even the standard DI gas system, run unsuppressed. I rarely shoot rifles, unsuppressed. I've tested every possible permutation of this discussion. And I've given you FACTS from personal first hand experience. You truly sound like you have never actually fired a suppressed gas gun. Or cleaned one after 50-100 rounds fired. That yer speaking from internet reads. That you beleive everything that adjustable gas block maufacturers say in their adverts.
 
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PBWalsh

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$240 gets you your lower. I’d be tempted if I had a bit more cash in the bank. These kind of offers make me think a .308 pistol would be useful.
 
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SkyScrapin

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Not much more benefit than a 300 blackout IMO and you can make them less annoying. 308 velocity is pretty anemic at that barrel length I know I was only getting about 2550 out of 16.5 308 an 11” would be close to 2300 I would think depending on the bullet.
I get 2700fps w/ 135g SMK. What projo you shooting?
 

Aescobar17

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Interesting, I appreciate the input, I’ll be matching it to a SiCo 7.62 Specwar can. It’s a heavy bitch alone. Will definitely research the 308 barrel length rating.

Would and adjustable gas block help tame some of the blowback or is that just trade off of having a short barrel?
I’ve ran a 12.5” with Specwar 762 a bit. Not much fun, but the Specwar tamed it as well as anything could. But in that combo it’s long and heavy anyways. Also Dead Air and Rugged warranty their cans to no barrel length restrictions. Personally I’d probably go Nomad for that use. More volume, but still shorter and lighter.
 

AK4900PA

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Every time the topic of short barreled .308's comes up people compare it to the 300blk. Obviously a 300blk is generally going to be considerably lighter, cheaper and more practical the ballistics are not even in the same ballpark. A 12.5" .308 is pushing a 175gr at similar speeds to what a 12.5" 300blk pushes a 110gr bullet. Far more energy on target with much higher ballistic coefficients.

I'm not going to defend the practicality of the .308 SBR though as it is a heavy pig. With a loaded mag and suppressor attached I'm looking at 12-13 pounds. If I'm walking any sort of distance into the woods I will likely grab my 12.5" Grendel 9 out of 10 times, but having options is never a bad thing.
 

bfoosh006

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$240 gets you your lower. I’d be tempted if I had a bit more cash in the bank. These kind of offers make me think a .308 pistol would be useful.

BTW... PSA now offers a 12.5" 308 upper as well.
 
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Yondering

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Basic gas law....it takes the path of least resistance....even if you magically find this ideal time to unlock the bolt....which you have absolutely NO way of knowning if yer doing, cuz the time btwn the primer igniting and the bullet leaving the barrel is about 1/1,000th of a second. The path of least resistance is the bore. THAT is where the gas/ crap is gonna flow. I've got adjustable gas on my .223 AR. I've fooled with its settings, found the point it just barely consistently cycles. That is CLOSE to that magical ideal moment. It STILL dumps a crapload of carbon into the receiver. (I've used both Seekins and Superlative Arms type adj gas) If 25-26 grains of 223 powder does that, 40+ grains of 308 will do it more. Best bet is to run 208-220gr heavy bullets subsonic. But as was said above, better to just get / build a 300BLK, than a 308.

Yer "magic moment" theory violates basic physics. There's no free lunch, here. Suppressors create back pressure. ALL of them, on every gas system. That backpressure exists WHILE the bullet is in the can, after the bullet has left the barrel. At that moment, the path of least resistance IS ** the bore.*** You suppress a gas gun, yer gonna get a ton of crap dumped in your receiver. Suppressor MANUFACTURERS WAY smarter than both of us have confirmed this. (Its still fun to do, anyway.) An adjustable gas block helps a little, but not much. Best avialable solution is a piston system....which STILL has the bore / chamber as the point of least resistance. I run those too, in 300 BLK, subsonic, and 223 supersonic. They gets WAY more crap in the receiver that even the standard DI gas system, run unsuppressed. I rarely shoot rifles, unsuppressed. I've tested every possible permutation of this discussion. And I've given you FACTS from personal first hand experience. You truly sound like you have never actually fired a suppressed gas gun. Or cleaned one after 50-100 rounds fired. That yer speaking from internet reads. That you beleive everything that adjustable gas block maufacturers say in their adverts.
Again, you know about half the story and just enough to be misleading to everyone who listens to you. You already proved in the other thread that you don't know when to stop talking; just let it go. Plenty of us here have hands on experience with this stuff that we actually understand, which you apparently do not. Also, look up the meaning of "straw man argument" since that's what you're attempting to do.
 
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Yondering

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I'm not going to defend the practicality of the .308 SBR though as it is a heavy pig. With a loaded mag and suppressor attached I'm looking at 12-13 pounds. If I'm walking any sort of distance into the woods I will likely grab my 12.5" Grendel 9 out of 10 times, but having options is never a bad thing.
While I agree with some of that, there's no reason at all a 308 SBR needs to weigh 12-13 pounds. Even my 18" is only 9 lb including the optic and an empty mag, and under 6 lb is achievable for an SBR without getting into crazy light race parts. If you ended up with a heavy 308, it's because you wanted to or you didn't pay attention to weight of the parts; it's not because they have to be heavy.
 

AK4900PA

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While I agree with some of that, there's no reason at all a 308 SBR needs to weigh 12-13 pounds. Even my 18" is only 9 lb including the optic and an empty mag, and under 6 lb is achievable for an SBR without getting into crazy light race parts. If you ended up with a heavy 308, it's because you wanted to or you didn't pay attention to weight of the parts; it's not because they have to be heavy.
I could easily drop off 2-3lb with a lighter optic, lighter stock and a lighter barrel, but under 6lb for a large frame AR is a fantasy. The weight I gave is with the suppressor which is 17oz including the mount and a loaded mag which is close to 2lb.
 
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garandman

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Again, you know about half the story and just enough to be misleading to everyone who listens to you. You already proved in the other thread that you don't know when to stop talking; just let it go. Plenty of us here have hands on experience with this stuff that we actually understand, which you apparently do not. Also, look up the meaning of "straw man argument" since that's what you're attempting to do.

That's just ad hominem. In other words you're showing you have no fact or logic to support your position. You just think that saying foolish things about *me* will earn you brownie points around here.



So let me put it in a single sentence for you…



Unless your "magic moment" to unlock the bolt happens after the bullet exits the muzzle of the suppressor, the entire system is back pressured and the gas will take the path of least resistance down the bore and back into the receiver... The obvious to everyone but you problem with that is that if the bolt doesn't unlock until after the bullet leaves the suppressor , then there is no pressure in the system to operate the bolt.

FACT. Proven by the fact that even piston systems get all sludgy in the receiver when shooting suppressed..
 
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TxWelder35

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Shot a HK 51k with a f/a trigger pack and a 4" Barrel. That thing was a riot. Welding gloves necessary for grip hand
 

Yondering

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Unless your "magic moment" to unlock the bolt happens after the bullet exits the muzzle of the suppressor, the entire system is back pressured and the gas will take the path of least resistance down the bore and back into the receiver...
I think you've misunderstood my objection to your comments. I didn't disagree with you that most of the gas comes back down the barrel. I disagreed with the part where you indicated to the OP that adjusting the gas system doesn't help; it makes a huge difference to tune the gas system correctly. Delaying bolt unlocking until more of the backpressure subsides goes a long way to making the gun work right and the whole experience more pleasant. There's no magic, except for those who just don't understand what's happening.

Besides the straw man arguments (if you don't know what that means: you're arguing against something different than what I said), you should probably do some studying of bolt timing. If your bolt opens before the bullet exits, you've got serious timing issues. Yes, suppressors cause back pressure, but not because the bullet is still inside. That's just another example where you don't seem to understand the full story, but are happy to tell people about it anyway.



I say all of this from personal experience; none of it is repeated from anything I've read or been told.
 
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AK4900PA

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It's not fantasy. Do some reading about lightweight AR builds, both 308 and AR15 size. Neither one needs to be heavy unless you want it to.
I've done more than a little reading along with some building. Show me a large frame AR with optic, ready to fire that's under 6lb. Id like to see the build sheet on that.

The 2A Armament rifles are about as good as it gets for lightweight AR's. Their XLR-18 weighs in at 6.75lb. You could drop ~1lb with a shorter barrel and handguard. So that's 5.75lb. Now add a lightweight LPVO and a loaded 10 round pmag. Now we're back up to 7.25-7.75lb.

You criticize the fact that my rifle will come in at 12-13lb and then follow it up with the fact that your rifle only weighs 9lb with an empty mag. Fill up that mag and add a suppressor. Guess what your now at 12lb.😉
 
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LeadZeke

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I built this one as a pistol. Loud, heavy and not much fun? 😂
Do it...! This one is built with a Bartlein from Craddock Precision. I may SBR in the future because the arm brace sucks!
What sucks about the brace?