Backpressure Problem in a Semi-Auto Rifle?

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Santander
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Backpressure Problem in a Semi-Auto Rifle?

Post by Santander »

I finally got my first suppressors out of ATF jail back in early December.
I got a Silencerco Spectre II and a SpecWar 762 and I've been really been enjoying them.

I bought the SpecWar 762 to use on 3 rifles.
  • AR-15 in 5.56mm (DI action)
  • Savage bolt rifle in .308 (not threaded yet)
  • Ruger SR-762 (piston action AR in .308)
I have been very impressed with how the SpecWar has done on the AR-15, both its suppression and how the rifle runs.
The Ruger SR-762 also did well for about 80 rounds or so, at which point I started having problems in the action.
I noticed that there was plenty of filth blowing back into the action and the magazine, coating the next rounds and I suspect that this may be contributing to the action fouling up (which I was not expecting from a piston action).

Anyway, I was wondering if a suppressor generally causes enough backpressure to do this.
I am thinking that every so often I will need to wipe down the BC group and spray it down with CLP to keep things working properly. What do you all think?
Are there any other considerations I should be keeping in mind when using a suppressor in a semi-auto rifle with a high energy cartridge such as .308?
I am not convinced one way or another that the root problem is related to the suppressor usage. It could be that this rifle needs to run wet. Maybe the fouling in the action exacerbated it, maybe not.

I'm looking forward to your feedback.
chickenlips
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by chickenlips »

Well your not alone with this problem. Piston rifles tend to suffer less from backpressure fouling than direct impingement type ARs. My experience with 556 suppressed is better functioning with a thirty caliber can over a 556 can. Bigger hole out the front makes less crud going in reverse. When you shoot 308 with 308 can you get a better gas seal bullet to bore hole in the can, thus higher pressure in the guns action. These rifles weren't originally designed to run suppressed. OSS suppressors tried to solve this problem with long helical internals that kind of look like jet engines. They achieved less back pressure but ended up with louder suppressors. On your piston AR you can possibly find an aftermarket smaller gas port. Which is what I did with my M1A. Other than that, get serious about cleaning that thing. Cause its gonna get nasty! Good luck!
aries14482
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by aries14482 »

With a suppressor there is almost no difference between a piston and a DI gas system. By far, the greatest majority of fouling is coming back through the chamber because of the suppressor. There is nothing you can do about this. Just keep a bottle of lube around and spray in the open action every thousand rounds or so and you'll be good.

Cans with greater volume will reduce this effect.

Maybe someone will make a barrel with an anti blowback device like that used on a tank cannon?
Santander
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by Santander »

Thank you for the confirmation, guys.
Now that I know what the issue is, I think there are ways to deal with it.

Part of the issue was trying to separate any rifle-specific issues with any additional things I need to take into consideration when using a suppressor. At first I wasn't sure if I had a rifle problem or a problem caused by the suppressor.
My first SR-762 had some serious issues. I sent it back to Ruger and they sent me a new rifle off of the line with all of the problems resolved. The new rifle has been running great, in fact, with and without suppressor (40 rounds total for that shooting session).
Yesterday's shooting session was more like 70-80 rounds (still not a lot of rounds) where most of it was with the suppressor... and it started exhibiting one of the issues I had with the earlier rifle. (Stovepipe while also picking up the next round - basically BC not coming back with enough energy to throw expended case clear before it starts picking up the next round. Result: Begins shoving new cartridge into chamber with an empty case above it sitting perpendicular and hanging out of the upper receiver. New case gets deformed and also bent at the neck.)
I still wonder if this rifle is any more sensitive to the fouling than any other rifle... but some of that will come to light as I put more rounds through it.

I really like the SR-762 (with upgrades to the trigger and stock). It's lighter and has a softer recoil impulse than my M1A. It's easier to scope as well.
I like shooting it with the SpecWar even more. Even softer recoil impulse and obviously less noise. Gone are the days that I shoot enough .308 that I can shoot with both eyes open... so this helps prevent flinching.
One nice thing that it offers is an adjustable gas port. It does have a setting where the port is closed, thus essentially turning the rifle into a bolt gun. With that setting the bolt will stay locked and the gasses should not foul the action. However, one of my goals was to use the suppressor with this rifle in medium range rifle event (Woody's Designated Marksman Match) and I'd prefer to have the quick followup of a semi-auto. The event in question requires somewhere between 80 and 120 rounds. So now the plan is to see how well my mitigation plan will work (wiping down the bolt and spraying CLP).
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MJF1911
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by MJF1911 »

On my SR762 with the Specwar I don't get any gunk on the bolt, just the cases from carbon that gets blown back through the chamber. I haven't had any reliability issues either. What ammunition are you using? Have you tried different ammunition?

When you changed the stock, did you change the spring and buffer?
SWR 556 Specwar, 762 Specwar, Octane 45HD, Spectre II; SilencerCo Osprey 45, Omega; AAC Pilot, 300TM; Ruger AC556
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Dy6078
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by Dy6078 »

Have a new SR 762, runs fine on standard ammo of any kind, finally received my tax stamp for my suppressor, rifle will not cycle with subsonic ammo, two different brands, different mags, with or without suppressor. Spoke with Ruger, " hmm it should work fine, try a different brand of ammo, and let us how it goes" tried different ammo, no go. tried all gas settings, no go Ruger suggested setting #3, no go. Anyone got ideas of how to solve this issue, oh runs fine with standard ammo with suppressor. Thanks !
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doubloon
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by doubloon »

Dy6078 wrote:Have a new SR 762 ... will not cycle with subsonic ammo ...
It won't.

If you keep cutting coils off the spring eventually you can get them to reliably eject but it will not then chamber another round.

Only two people I know of have every claimed to get subsonic ammo to cycle a in a 308 AR-10ish. One was some guy with a special POF rifle who said they'd be available in 2 weeks but I never heard from him again. Another was a guy who said he had a special super secret subsonic load recipe for 308 that would cycle that he said he'd publish and I never heard from him again either.

I don't know anyone who knows how to get a 308 AR-10ish to cycyle subsonic ammo but maybe someone will respond to this post with the answer of my dreams.

ETA: Maybe give this stuff a shot ... not with your suppressor ... at a little over a buck and a half a round https://www.midwayusa.com/product/10195 ... -box-of-50
http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
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Dy6078
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by Dy6078 »

Spoke with Ruger this morning, they want the rifle back, "it should work with the subsonic ammo" so we can inspect, evaluate, and correct the issue, about a week turn around, thinking about sending it back, it is useless at the moment with subsonic ammo
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John A.
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by John A. »

I don't care what your chart says
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fishman
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by fishman »

I don't know anyone who knows how to get a 308 AR-10ish to cycyle subsonic ammo
a pistol length gas system and a rifle length barrel. Just don't run supers through it
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5.56 form 1:
http://www.silencertalk.com/forum/viewtopic.php?f=10&t=141800&p=955647#p955647
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doubloon
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by doubloon »

fishman wrote:
I don't know anyone who knows how to get a 308 AR-10ish to cycyle subsonic ammo
a pistol length gas system and a rifle length barrel. Just don't run supers through it
I've thought about trying this myself but was wondering if I could restrict enough gas using an adjustable block to make supers bearable.
http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
http://www.youtube.com/watch?v=8CrOL-ydFMI This is Water DavidW
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fishman
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by fishman »

doubloon wrote:
fishman wrote:
I don't know anyone who knows how to get a 308 AR-10ish to cycyle subsonic ammo
a pistol length gas system and a rifle length barrel. Just don't run supers through it
I've thought about trying this myself but was wondering if I could restrict enough gas using an adjustable block to make supers bearable.
My guess is that it'd be too finicky with different loads unless it was way over gassed
300 blackout form 1: http://www.silencertalk.com/forum/viewtopic.php?f=10&t=137293

5.56 form 1:
http://www.silencertalk.com/forum/viewtopic.php?f=10&t=141800&p=955647#p955647
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doubloon
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Re: Backpressure Problem in a Semi-Auto Rifle?

Post by doubloon »

fishman wrote:...
My guess is that it'd be too finicky with different loads unless it was way over gassed
There are already plenty of finicky gas rifles out there I don't need to make my own then.
http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
http://www.youtube.com/watch?v=8CrOL-ydFMI This is Water DavidW
Complete Form 1s http://www.silencertalk.com/forum/viewtopic.php?f=10&t=79895
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