AR10 over-sprung?

[snarkscarbine]

I think I know what my problem is, I just need some insight from people more knowledgeable than me.

I've built a decent number of small-frame guns that all run really smoothly suppressed. Obviously .308s are a different critter, but I know a decent amount about how the system works.

Here's the parts list:
M5 receivers
Faxon Big gunner 16" mid-gas
Faxon bcg
H3 LR308 buffer
Vltor carbine buffer tube
Tubb AR10 spring
MBT-2s
Radian SD CH
Superlative gas block
Rex Silentium can


I built this to be my new thermal rig, and it's going to be shot ~100% suppressed with the can mentioned above.

The problem I'm running into is that tuning the gas has been a b*tch. Getting it to lock back reliably anywhere in either sector (constriction/bleed off) of the gas block's function has left me ejecting around 2:00 with pretty rough cycling. For context, this is my first AR10 build but not my first time shooting one, so I'm not just surprised at how much more recoil it has than a 223.

Am I correct in thinking that the Tubb spring rate is just too beefy when compressed down into a carbine buffer tube, and that's what's making me have to gas the fvck out of it? What spring should I get?

 

[Mike_in_FL]

I've had no problem using that spring before but I was using the Slash heavy carbine buffer so I don't know how the weight compares to yours. It sounds like you're putting too much gas into the system. I would just close the AGB with your suppressor firing single rounds and just open it until you lock back on an empty mag and a click or two more for reliability.

 

[snarkscarbine]

Yeah that's what I did. I've tuned gas on numerous rifles before and never had a result like this. Like I mentioned above the only point at which it locks back reliably has it ejecting at 2:00 and running rough.

@Mike_in_FL is yours in a rifle-length buffer tube or carbine?

Buffer is 5.3oz

 

[Mike_in_FL]

It was a carbine, essentially same extension as yours but the Magpul version with an UBR. I used IIRC the 1st on this page so 5.5 oz. http://heavybuffers.com/ar10carbine.html

This was on an actual Armalite AR10 older B model but I sold it awhile ago. I always was of the opinion that mid-gas large frames was too violent but apparently they port them to run on weak ball 7.62 so they have to be over gassed to an extent.

I have a few SA gas blocks and have never seen one fail but it sounds like it's not regulating the flow well enough maybe. At this point the only thing I could try and suggest is either a heavier buffer or JP SCS or an adjustable gas key for the carrier. You could always pin gauge the gas port in the barrel to determine size but I would think it's just going to be no more than .093, and even if oversized I think the gas block should handle it.

 

[bfoosh006]

I don't "think" the Tubbs would be to much spring.

Spring rates as measured by David Tubbs ... NOTE rifle length buffer tube used.


A2 (old) - 7.6lb closed, 14.5lb open
A2 (new) - 9.1lb closed, 16.7lb open
Tubb Flatwire - 10.5lb closed, 16.3lb open
Tubb .308 Flatwire - 13lb closed, 16.7lb open
SpringCo White ? (standard) - 8.3lb closed, 16.3lb open
SpringCo Red (extra) - 10.5lb closed, 18.4lb open
SpringCo Orange (increased) - 13.9lb closed, 24.8lb open

And while none of my Large Frames are suppressed... I run 308 Tubbs in all of them.

DPMS pattern buffer tubes ( AR15 carbine buffer tube ) with short 2.5" buffers ( KAK 5.3oz 2.5" )
Setups like yours .. Vltor length / ( Armalite Pattern ) with 3.25" buffers ( 5.4oz AR15 carbine buffer )
And Rifle length stock setups KAK 9.3oz 308 rifle buffer.

Do you happen to know your gas port size ? I realize the adj GB should be limiting the gas... but....

And I understand it is to be fired 100% suppressed.... but how does it function without the suppressor ?

I can't help but wonder if the 308 suppressor design is overwhelming the Adj. GB.

 

[newguy2k3v2]

Do you have an actual carbine length buffer tube or a 308 length carbine/a5 tube?
Is your h3 buffer the standard length or one of the shorties to work with a 308 and ar-15 carbine buffer tube?

 

[snarkscarbine]

Do you have an actual carbine length buffer tube or a 308 length carbine/a5 tube?
Is your h3 buffer the standard length or one of the shorties to work with a 308 and ar-15 carbine buffer tube?

Standard Carbine buffer tube w/shorty 2.5" LR308 style H3 buffer.

 

[newguy2k3v2]

Standard Carbine buffer tube w/shorty 2.5" LR308 style H3 buffer.

I think the tubb spring is supposed to go in a ar10/A5 length tube with a standard buffer. Iirc setups like yours with the shorty buffer have their own specific springs which are a little shorter or have fewer coils.

 

[mudpig]

The Tubb spring should be good to go. Your buffer might be a little on the light side tho. I've got an AR10 built with M5 recievers/DPMS guts and adjustable gas block. I'm using the Tubb spring (uncut), but I'm using one of the hydraulic buffers. Not sure of the weight of it, but it's heavy and soaks up some of the violence going on. My rifle will cycle reliably, kicking brass out at about 4 oclock when dialed in with the gas block. Barrel on mine is mid-length and can't remember port size off the top of my head...it's either .086 or .093

KYNTEC CORPORATION AR-15/M16/308 AR RECOIL BUFFER | Brownells

Designed for law enforcement, sportsmen, and competitive shooters alike, these hydraulic buffers help reduce recoil, while diminishing wear and tear on parts. The sealed hydraulic cylinder is made of 17-4 stainless steel and features a nickel-plat...

www.brownells.com

 

[mtrmn]

I put together a PSA 308 mid length 16" kit. Never could get the gas adjusted right. Either what I saw as overgassing symptoms, or it wouldn't lock back. Had a few case head separations also-with what should have been LIGHT loads. Added a Sprinco orange spring, a Kaw Valley 5.6oz buffer, (all for the carbine length buffer tube) etc and it all seemed to help but just not enough. I wanted to lower the weight some so I installed a Faxon pencil bbl. Same problems as before. Finally got some headspace gages and checked it. The gun would fully close on a NO-GO gage. I ordered a Lantac bolt and all problems went away. Headspace is now correct and it was easy to adjust the gas.

 

[snarkscarbine]

k I've been reading over on 308AR and apparently using a standard 11.75" mid-length gas tube (which I am) can cause this problem on barrels drilled for a 12.0625" Armalite-length tube (which Faxon's Mid-gas barrels are). Got one of those ordered, and we'll see if that helps things.

 

[mtrmn]

k I've been reading over on 308AR and apparently using a standard 11.75" mid-length gas tube (which I am) can cause this problem on barrels drilled for a 12.0625" Armalite-length tube (which Faxon's Mid-gas barrels are). Got one of those ordered, and we'll see if that helps things.

So, even though my 308 runs good, now I'm wondering if it could run even better. Off to do some measuring......more time and $$$$.

 

[mtrmn]

So, even though my 308 runs good, now I'm wondering if it could run even better. Off to do some measuring......more time and $$$$.

Yep turns out I have the 11.75" midlength gas tube which, when coupled with my Faxon bbl, comes up too short. So now I need to find an Armalite-length 308 gas tube of 12.0625" length. Thanks for the lead, Snarks.

 

[GONE BAD]

Pretty interesting. I went to armalite to get their gas tube for a DPMS pattern rifle. I changed the barrel and in the reviews I learned that this was what I needed. Must be fairly common to some extent. In my case I was going to rifle length but still needed Armalite's longer tube. The supplier of the barrel wasn't aware of this. Sometimes reviews pay off

 

[mtrmn]

Update on the quest for Armalite-length gas tubes. Armalite no got the 12.0625" (carbine length for the AR10 according to Armalite) gas tube. Far as I can tell nobody else has that Armalite brand tube either. However I did find another brand - LBE Unlimited - Part Number ARGT308. Same length that I need. Did a DuckDuckGo search for that part number and found lots of them in stock. Priced anywhere from about $17 to $29.

 

[sinister]

For a dedicated, 100% of the time suppressed rifle you should be using a rifle-length gas system / gas tube.

Once you get your Armalite 30-cal mid gas tube you'll still need to monkey with the gas setting for different bullet weights through your suppressor. The more efficient your suppressor the more you'll need to accommodate the higher system back-pressure -- trickier to do with short(er) gas systems and higher port pressure.

You should change your receiver extension tube to a Vltor A5 with an XH buffer. The Tubb spring is correct and preferred.

Faxon Big gunner 16" mid-gas

H3 LR308 buffer

Vltor carbine buffer tube

Tubb AR10 spring

Superlative gas block

Rex Silentium can


... it's going to be shot ~100% suppressed with the can mentioned above.

The problem I'm running into is that tuning the gas has been a b*tch. Getting it to lock back reliably anywhere in either sector (constriction/bleed off) of the gas block's function has left me ejecting around 2:00 with pretty rough cycling.

Am I correct in thinking that the Tubb spring rate is just too beefy when compressed down into a carbine buffer tube, and that's what's making me have to gas the fvck out of it? What spring should I get?