View Full Version : Short Stroking with Vltor A5
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?
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
05-24-16, 22:11
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...
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
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
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
05-25-16, 00:11
10.3-10.5" car gas barrel should have a gas port of .071-.073"
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.
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.
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.
yeah, running some 5.56 lake city xm193 is a good idea. However, assuming you don't have a gas leak, going with an A5H0 or A5H1 buffer is definitely where you need to go next (cheap and doesn't require permanent modification). Carbine action system and the A5 action system are completely different beasts so don't compare the H2 (carbine) with the A5-H2 (A5). best of luck. let us know.
Yeah don't open up your gas port.
Just use a suppressor.
jerrysimons
05-25-16, 17:32
Good advice above. You did go a hair under on port size for that length of barrel when using .223. Crane spec is for 556 only.
Yes you were "limp writing the rifle" holding it limply but it seems you know to do that on purpose when testing function so keep testing it that way for a better margin of function. If every thing is built properly try the lighter a5 buffer weights equivalent to a carbine H2 and H1. If you still have trouble with .223 pressure ammo, see where the LMT Enhanced carrier gets you. It can allow the action to cycle with less pressure by reducing the work it takes to extract a case.
If none of that works you have a suppressed only barrel! Or drill the gas port.
That's true about the LMT carrier. The difference between the LMT and the standard carrier is one click on the SLR adjustable had block. I could close the gas one click more, using the LMT
Thanks for the replies. You suggested a couple of options I hadn't thought of before. I suppose the 14.5" barrel with .0625" gas port has more than enough gas then since I can use an A5H4 buffer doing the same function test.
Yeah don't open up your gas port.
Just use a suppressor.
No doubt! Do you know how hard it is to get a short barrel that's NOT over friggin gassed?
It's a cut down M4 barrel, nothing special. If you want one for suppressed only it would be better to get one cut down and leave the port at .0625"
I suppose the 14.5" barrel with .0625" gas port has more than enough gas then since I can use an A5H4 buffer doing the same function test.
The reality is that you need more than "just enough" for a reliable combat weapon. I would suggest the planned use of this rifle be the deciding factor on upping the port or not.
I'm not planning on using a suppressor with this one.
I want it to perform with the function test I did but I don't want it to be overgassed as the bolt rings loosen, the gas port erodes, and the action spring weakens.
Opening the gas port seems like the best idea but I don't know how much of an increment would be reasonable. The next drill size is 1.8mm .071". I don't want to remove the FSB a bunch but I don't want to make it too big either.
I'm not planning on using a suppressor with this one.
I want it to perform with the function test I did but I don't want it to be overgassed as the bolt rings loosen, the gas port erodes, and the action spring weakens.
Opening the gas port seems like the best idea but I don't know how much of an increment would be reasonable. The next drill size is 1.8mm .071". I don't want to remove the FSB a bunch but I don't want to make it too big either.
Why don't you try an H1 and H0 buffer, first, and then go from there? I also suggest the Sprinco Green spring. That spring seems to make things function more robustly than the one Vltor comes with, even though it's "stronger". I do not know how or why, but that is my experience. My DDM4 will run Tula with an H4 buffer unsuppressed with that spring, lol
I thought about that, but since there isn't much of a difference between the A5H2 and A5H4 I don't know if it would change much. I probably could make an A5H1 or A5H0 from my other buffers so I won't be out any money trying it. I'm using a new Colt spring, so everything should be good there. A stiffer spring should cause more short stroking.
I thought about that, but since there isn't much of a difference between the A5H2 and A5H4 I don't know if it would change much. I probably could make an A5H1 or A5H2 from my other buffers so I won't be out any money trying it. I'm using a new Colt spring, so everything should be good there. A stiffer spring should cause more short stroking.
At least try the A5H0. Or whatever. Just do ANYTHING and EVERYTHING before opening that gas port!
Why don't you try an H1 and H0 buffer, first, and then go from there?
Yeah, that seems like a no-brainer to me.
jerrysimons
05-26-16, 08:55
I bet you an A5H0 would work, grab a LMT E carrier and I bet you could use a A5H2 without problems.
I don't think you can use the term overgassed when describing a worn together gun Tthere should not be that much of a difference between new and worn in to get you from gassed right to overgassed.
I believe a larger gas port matched to a heavier buffer is more reliable on shorter barrels. I'd open the port to .073" and be done with it.
for the third time, buy an A5-H0 buffer and a sprinco green spring. i would be shocked if you still had issues after doing this. :)
I believe a larger gas port matched to a heavier buffer is more reliable on shorter barrels. I'd open the port to .073" and be done with it.
This was my original thinking and probably what I'll end up doing in the end.
I will try it with a lighter buffer.
Oh if you are never going to suppress it then you shouldn't be concerned about gas if lighter buffer doesn't work.
OP,
I would also suggest trying different buffers along with the LMT E-Carrier. I've got a 10.3" barrel with .070" and run an A5H2, BCM rifle spring, and an E-Carrier and get lockback with .223-spec Federal ammo.
Just a small update. I did go shoot it with XM855 and Mk318 Mod 0. It functioned just fine with an A5H4 buffer but still wouldn't pass the .223 "limp wrist" test. I made an A5H0 and A5H4 from two A5H2 buffers. It does pass the test with the A5H0. I'm either going to leave it how it is or open the port to .0730" so it passes the test with the A5H4.
jerrysimons
06-25-16, 15:51
Just a small update. I did go shoot it with XM855 and Mk318 Mod 0. It functioned just fine with an A5H4 buffer but still wouldn't pass the .223 "limp wrist" test. I made an A5H0 and A5H4 from two A5H2 buffers. It does pass the test with the A5H0. I'm either going to leave it how it is or open the port to .0730" so it passes the test with the A5H4.
Why would you open up the gas-port just to use a heavier buffer?? There are three stages of buffer in between the two that you have now and with all the components of 2 A5H2 buffers you can make all of the possible A5 buffer combos to test further.
My reason for wanting to use the heaviest buffer possible is that the extra mass should be more reliable. Also, all of my other uppers work with A5H4s, so I could have parts commonality and use any upper on any lower if I had to.
The only buffer weight I haven't tried yet is the A5H1. A5H2 and A5H3 also failed.
A heavier buffer may help if you're over-gassed. But I wouldn't hog out the port just to run a heavy buffer. Put in a -0 or a carbine and see how it runs. Less gas and light buffers = very light recoil.
A heavier buffer may help if you're over-gassed. But I wouldn't hog out the port just to run a heavy buffer. Put in a -0 or a carbine and see how it runs. Less gas and light buffers = very light recoil.
Exactly.
BufordTJustice
06-25-16, 23:35
My reason for wanting to use the heaviest buffer possible is that the extra mass should be more reliable. Also, all of my other uppers work with A5H4s, so I could have parts commonality and use any upper on any lower if I had to.
The only buffer weight I haven't tried yet is the A5H1. A5H2 and A5H3 also failed.
This is a perfect situation to implement an LMT enhanced carrier. You'll likely be able to keep the A5H4 buffer as well.
I have an 11.5 bcm with lmt carrier, green spring and A5H4, works great
My newer custom upper is a chopped FN barrel down to 10.8" with a 069, maybe 0695 port. I started with a green spring and it would cycle pmc
with an A5H4 and A5H3 but was having some failure to lock back when dry (sionics np3 carrier), I went down to a A5H1 and it seemed to help a bit but
was still getting some failure to lock back. Even though the green sprinco is not supposed to be any stiffer or stronger (just longer lasting), I saw a thread here
from a few years ago about guys with A5 lowers and SR15 uppers having failure to lock back, some reported success by going back to a mil spec rifle spring, this
worked for me as well. So the green spring must be stiffer/etc
I didn't try the lmt carrier from the bcm gun, wish I would have, still might later
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