Short Stroking with Vltor A5

[pezboy]

I just put an upper together with a Colt M4 barrel cut to 10.3" with a .0700" gas port and standard Colt BCG. When test firing (pointing the rifle down while holding it loosely), it was failing to lock the bolt back 1/2 of the time. This wad with an A5H2, A5H3, and A5H4 buffer and multiple mags. The bolt does pick up a fresh round when cycling. If I hold the rifle firmly it does lock back. Am I possibly "limp wristing" it? Do I need to open the gas port up more?

[Ryno12]

I just put an upper together with a Colt M4 barrel cut to 10.3" with a .0700" gas port and standard Colt BCG. When test firing (pointing the rifle down while holding it loosely), it was failing to lock the bolt back 1/2 of the time. This wad with an A5H2, A5H3, and A5H4 buffer and multiple mags. The bolt does pick up a fresh round when cycling. If I hold the rifle firmly it does lock back. Am I possibly "limp wristing" it? Do I need to open the gas port up more?

Helps to know what ammo you were using.

[daddyusmaximus]

Sounds like less powerful PMC Bronze .223 ammo. You could open up the port then use an adjustable gas block to dial it in...

"limp wristing" is more of a handgun thing. I've never known it to be a problem on a rifle. Of course, there's always a first time...

[ace4059]

What ammo?

.070 sounds like a small gas port for a 10.3" barrel, combined that with the A5 buffer assembly it is going to make it worse.

Have you tried it without the A5?

If you are going to keep the A5,
I would drill it to .073" but it might need to be drilled as big as .076"
DD barrels are over gassed and I think they run .081-.083ish

[MistWolf]

Put a few drops of lube in the exhaust holes of the carrier, grab a couple hundred rounds of full power ammo, hold it firmly and shoot it. Don't open any gas ports. Just shoot it. As long it doesn't fail while holding it firmly, keep shooting it. A bit a break in might solve your problem

[joeyjoe]

I agree with mist. Do NOT open up the gas port unless you have no other option. First off all, as others have said, we need to know the ammo. Secondly, go purchase an A5-H0 buffer (3.8 oz) and/or an A5-H1 buffer (4.56 oz.). Also, i run Sprinco green action springs in my A5 RE. The green springs are a bit more robust than the stock A5 springs and, periodically, have been anecdotally attributed to increased reliability.

[Robb Jensen]

10.3-10.5" car gas barrel should have a gas port of .071-.073"

[Todd.K]

It's sized just under 556 ammo and carbine H buffer. You have a heavier buffer and may want to run 223 ammo.

I've found that holding the rifle loosely replicates a dirty gun pretty well. I'd prefer it to run a little harder when clean than choke when dirty. Also fair weather just barely runs may choke in the winter. I'd go up a step or two until it locks back all the time.

[bruin]

Go for the low hanging fruit first, so to speak. Make sure the gas block is aligned. Check for signs of leakage at the block. Check for a loose gas key. Make sure the gas key is not obstructed. Check that the end of the gas tube is aligned with the gas key and doesn't bind. With the upper only and held upside down, tilt it and see if the carrier (sans charging handle) slides freely down to meet the barrel extension. See if the carrier binds at all as it cycles. If using a stock rifle spring, check the free length. Make sure the bolt catch moves freely and engages properly. Verify that your mags are known good using other guns.

[pezboy]

I was testing with Federal .223 55 gr (purposely using this as a "weak" ammo, not sure if it is any weaker than my XM855 and Mk318). The bolt carrier slides freely and everything was lubed well. I'm not sure if there are any gas leaks. The bolt rings are tight since they are new so I know it could be a little sluggish. The bolt catch seemed to wiggle more than usual but I tested it with another lower (same buffer system) and it functioned the same.

I don't want to open the gas port up if I don't have to but am willing to if that is what it needs. I'm going to stick with the A5 as well. MK18s are .0700" gas ports with H2 buffers which I realize is different than the A5 system but I wouldn't think I would need an A5H0 or A5H1. I think I will probably put some more full power rounds through it to break it in and then test it again with the .223 later.