Dipped my foot into the 308 AR build world and not really liking it much so far, but I’m in now so it is what it is. The problem is short cycling. Here’s the parts list:
[ul]
[li]Aero Precision M5 receivers[/li][li]Aero Precision phosphate BCG[/li][li]Aero Precision 16" SS barrel with midlength gas system and gas port of appx. 0.075"[/li][li]Aero Precision LP gas block[/li][li]Unknown mfg mid length gas tube (from the parts bin, but it is the same length as all my other mid length gas tubes)[/li][li]BCM charging handle[/li][li]AAC 51t flash hider[/li][li]Misc lower parts with G2S trigger[/li][li]Vltor RE-10 receiver extension[/li][li]H3 AR15 buffer (and later a carbine buffer)[/li][li]Armalite AR10 operating spring (p/n EA1095)[/li][/ul]
I think that covers all the parts that could possibly effect the cycling of the gun. Now, I just built the gun and test fired it today and all I had was Tula ammo. Yes, it sucks and it’s really weak, but I’ve had good luck with it in an M&P10 so I thought the same would be true here. First I had the H3 buffer in it, and empties were dribbling out, landing almost right at my feet. Some didn’t make it out and they would cause a stoppage as it seems like the extractor tension is pretty high and it was not wanting to let go of the rim. The only other buffer I had with me to try was a carbine buffer so I threw it in and it actually seemed to cycle better. It still was not locking the bolt back on an empty mag though.
Normally I can troubleshoot an AR15 pretty well, but the 308 is a whole new can of worms and things aren’t standardized as you know. A few things stand out to me after looking the gun over.
[ul]
[li]First, it seems like the buffer spring is awfully stiff. I compared it to a factory M&P10 as well as a Colt LE901 and it just seems a good bit stiffer. I thought I had the right combo, but maybe not?[/li]
[li]The next thing is that I’m not finding anything binding but the bolt is pretty tight in the carrier. It’s well oiled, but maybe it will break in some and that will help? I don’t normally need any break in on AR type rifles, but what do I know about 308 AR’s…not much. It does seem a little bit better when cycling by hand after just a few rounds through it.[/li]
[li]Finally, I think there’s something funky with the gas tube length. It protrudes into the receiver about 1/4" less than the other 308 rifles I compared to. The gas tube is standard AR15 mid length. I did my best to measure the gas tube on the M&P10 (tough with the gun assembled) and it’s about 1/4" longer than an AR15 mid length gas tube. Is the lesser engagement with the gas key on my build going to impact the operation or is most of the energy for cycling being supplied before the carrier moves back enough to disengage the gas tube, even being 1/4" short?[/li][/ul]
I need to get some reloads ready to go and when I do, I’ll be shooting them. Maybe I shouldn’t even worry about it cycling with Tula, but this is really just a range gun and I’d like it to run on the ‘junk’ due to cost savings. I figure that Aero ports the barrels to run the weak stuff so I’m a little surprised to have this problem.
First thing I would try is a dpms 308 carbine sling and buffer. Being that the S&W is somewhat proprietary I wouldn’t count on that as a comparison. Even if it doesn’t work exactly right due to the vltor tube length you’ll know if the spring tension was some of the issue. Then you can work on a spring buffer combo that works. If it doesn’t change you know to look somewhere else.
Have you read through the AR10/.308. Build info thread yet? Might be some information there. I don’t know for sure but I was under the impression that the .308 builds needed a .308 specific buffer. Can anyone confirm?
Finally, I think there’s something funky with the gas tube length. It protrudes into the receiver about 1/4" less than the other 308 rifles I compared to.
Ding ding ding ding. That’s causing an early drop of gas pressure back into the BCG.
You have the correct buffer lengths for the RE tube, since you’re using an Vltor RE-10/A5. Standard AR15 carbine-length buffers work with this tube in the 1993-forward SR25/AR10’s.
You need an ArmaLite Ar10 carbine gas tube I think. Check to see if your gas block has been moved forward (not likely). When you remove the gas block, please ensure that the ports are aligned center-to-center.
I’m using the Vltor RE-10 receiver extension, so the short DPMS carbine buffer will not work. If there is enough gas to drive the bolt carrier fully rearward, it would stand a good chance of damaging the receiver since the buffer would be too short. There are ways around that, including swapping a standard carbine receiver extension on, but I really think the combination I have is correct.
I’ve read a good bit there and elsewhere. There are a few combinations of buffer system parts that work. I’m using the RE-10 receiver extension specifically so I can use AR15 length buffers. The DPMS LR308 carbine buffer is shorter so it can work in an AR15 receiver extension.
I checked the gas block and it was lined up perfectly with the port in the barrel. I’m using a standard AR15 mid length tube which measures 11.75" long. Best I can tell, the Armalite tube you’re referring to is 12 1/16" long. That might work, I’ll have to do some more measuring.
I REALLY like Aero Precision components. My sentimental and artistic taste favor their old logo rather than the stylized “A”.
3 things: the Tula may be weak stuff for your particular carbine; you may need the 1/4-inch longer Armalite 7.62 gas tube rather than a 5.56 middy tube; and last, yes, your spring may be too stiff for your application.
The combination of low power curve ammo, a gas tube a skosh too short, and a stiff spring all work to short-stroke your weapon.
After looking at this more I’m almost certain the gas tube is the problem. I don’t know why AP would make an LR308 pattern barrel that needs an Armalite gas tube.
Any recommendation on where to get the Armalite tube? Armalite wants $16 to ship a gas tube…hoping a stocking vendor will have reasonable shipping charges.
Huh…AP says it should use an AR15 gas tube. Barrel must be made wrong but I still plan to just get the Armalite gas tube and be done with it…sick of dealing with LR308 parts not fitting when AP says they should. This isn’t the only problem I’ve had with their 308 stuff and part fitment.
A friend’s cousin had the same issue (short stroking every round) with his Blackthorne (UGH, if that alone doesn’t scream problems, I don’t know what will) complete LR-308 upper. The idiots drilled the gas port at an AR-10 pattern rifle length location and put a standard AR-15/LR-308 rifle length gas tube in there.
I’d be interested to hear what AP says about it. Is the position of the gas port on your barrel the same as on a 5.56 mid-length barrel? If it is out of spec (to where AP intended it to be) have it sent back.
Armalite and Aero built several hundred AR-10s for California sale (not being on the by-name banned list). Until their M5 series everything was Armalite spec.
I have found several accounts of this scenario with a short gas tube causing short cycling on lr308 rifles. I’ll measure to confirm later but I believe the gas port and shoulder are at the mid length position for an AR15. From what I gather, lr308 barrels that use ar15 gas tubes have the port moved about 1/4" back typically…that could be wrong but it makes sense.
From shoulder of the barrel extension to the gas block shoulder is 8.75", which is exactly the measurement for a mid length AR15 barrel. That’s why the AR15 gas tube is coming up short.
And the 308 barrel extension is longer than the AR15, requiring a longer tube.
The 308 also has a more offset between the bore and gas key, so requires a special gas block, gas tube with more offset or just tweaking a standard tube to fit.
I got the Armalite tube in there. Now it has what seems like the standard tube protrusion into the upper. And the tube fits the offset from the bore to the gas key perfectly, where the AR15 tube really needs a little tweaking.
Functionally, there was an improvement. With everything the same as it was with the ‘wrong’ gas tube, the gun seems to be functioning fine now. Still a soft shooter in terms of secondary recoil, but it locks back after the last round now.
Interestingly enough I am dealing with the EXACT same issue: not enough power to extract the round, let alone lock back on empty.
Tried different springs, buffer weights , gas-settings (adjustable), no dice.
(Did not really spend much time though, kinda taking a few minutes here and there between jobs, running out back.)
Set up: M5 receivers, VLTOR A5 RE, AR10 spring, H3 buffer, SLR adjustable gas-block withmid-length gas-tube, 18" Mid RAINIER barrel…
Standard wisdom seems to be that “…Many if not most AR15 parts are interchangeable with the AR308 platform…”…BUSTED!
Reality says some may be, most are not. And that’s a fact.
So yeah, I will try the AR10 gas-tube as well, see what happens. Really interesting - have not seen this mentioned elsewhere, so i that indeed is the case this will make a nice addition to the 'build"-thread, fer sure!
One thing I am starting to realize - getting quality parts is even more important than with the AR15 platform, that, and one has to be willing and prepared to solve a few puzzles, which is kinda the fun part and well worth it - have no doubt by the time it’s all up n running it will hold it’s own just fine.
Wondering if i can get some clarification.
In your specs you list it as a ‘mid-length’ system.
The AR10 tube shown as purchased from Rainier to remedy the issue is shown as a ‘carbine gas-tube’.
So the AR10 carbine gastube is longer then the AR15 midlength tube?
This does not make sense, really. What am I missing - is the word carbine used here very loosely?
PS - in my (midlength gas) build I use a standard AR15 mid length gas tube, fit was no problem across the board.
EDIT:
Went ahead and put the ruler to it:
AR15 mid upper (Noveske N4) - gas-tube ‘protrudes’/clears exactly 1 1/16" into receiver.
The exact same is measured from M5 (Aero 308) upper with mid-length gas tube.
Gas-key on both 5.56 (M16) and 308 BCG is identical, same location, same size etc.
leading me to believe (in my case) gas-tube is a non-issue.