When assembling a new M4 rifle I noticed short stroking, either not picking up the next round or not locking on an empty magazine. All the builds used new upper receivers.
The DD barrel I used had a .058 gas port, I opened it up to .063; it still short stroked.
I built a rifle using a used Colt 6400 barrel with 1k fired cut down to 14.5", the gas port was .063, I cleaned out the gas port. It still short stroked.
I tore down and reassembled a 14.5" Blackthorne kit, it wouldn’t lock open on an empty magazine, it was AFU. The gas port was actually .063 if you all can believe it.
Instead of shooting the rifle loose I just cleaned and lubed the BCG and upper well after the initial test fire. I then worked the BCG in the upper by hand about 1k times, then cleaned it thoroughly again. The rifles all ran fine.
It is the first thing I do to all new rifles with problems if obvious defects aren’t detected. I’m not made of brass you know.
So you’ve assembled 3 rifles and they all short stroked on you.
You don’t really give much information.
You shouldn’t need to dry cycle a new rifle 1000 times for it to function properly.
You think you should be assembling rifles?
I’ve assembled about 11 rifles so far, I’ve just found the three that short stroked just needed to be broken in, without spending $300.
I guess the next thing you’ll tell me is I shouldn’t fire 1k rounds before I trust my rifle…
Also, I test rifles with Wolf ammunition. The last rifle I put together, a BCM mid length with a DD BCG, fired just fine, as well as most rifles built to function all ammunition (hogged out gas ports). The rifles that short stroked had one thing in common, .063 gas ports, which I believe I mentioned in the 1st post. If the rifle did not function with Wolf, a little hand lapping made sure it did. If it will shoot Wolf, it will shoot ANYTHING.
I wasn’t too concerned, the worst that could happen would be the gas port would be in spec.
The gas ports in several Stags and Bushmasters barrels I’ve built were between .073 and .078, from the factory. I won’t lose sleep from drilling the gas port to spec.
.062 is in spec as far as I know.
Personally I wouldn’t be touching the gas port, especially with out checking other parts first.
Your odds don’t look too good.
11 rifles built, 3 malfunctioning rifles.
You need to include more information.
Buffer?
Action spring?
BCG? Gas key loose?
Gas tube interfacing with the gas key properly?
FSB or gas block installed correctly?
In all fairness, did you assemble the BCM upper? (Install barrel, gas tube, FSB)
Also, do you know what the gas port size on the BCM upper is? I don’t think they run overgassed. (At least mine doesn’t)
I wouldn’t use wolf ammo to break in a rifle its too damn weak.
Most BCG’s will loosen up after 500 rds. You start out with full power 5.56 pressure ammo if it short strokes with 5.56 then you know some thing is wrong. If it works fine then you can try the lower powered stuff. Drilling out the gas port would have been one of the last things I would have done. Start with the simple stuff first then work your way up its called process of elimination.
14.5" & 16" gas port spec is .063, it will not change. Google is your friend.
I know what the spec is, .058 is not the same as .063, issue squashed.
Why would I waste money firing 500 rounds when I can work the BCG by hand?
The three weapons that short stroked all had in spec gas ports, breaking in the BCG/upper by hand solved the problem.
I’m trying to help people here. If your NEWLY BUILT gun short strokes, try the above if everything else looks fine.
The BCM was test fired at the factory, I would expect it to function, I was the first to fire the three that short stroked, all had new BCGs and uppers.
After hand cycling all three functioned fine with Wolf AND spec gas ports.
It really isn’t important, what is important is a person searching google finds this thread and tries a simple method before paying a gunsmith or sending a gun back.
The gas port specification is set by the amount of pressure the ammo produces. The TDP took ammunition into consideration when it was written. A gas port isn’t necessarily out of spec if the gun doesn’t fully cycle shit ammo, nor should a quality gun be judged on its ability to fully cycle wolf. BCM stands on its own, not because one rifle cycles weak ammo, but insofar as it follows the TDP.
This thread would be confusing for a newb to follow as it stands.
Post #1 is missing a ton of information.
I see now that you’re tuning your weapons to cycle low pressure ammo. It’s usually inexpensive ammo and many shooters like it for its economy.
There are also many shooters who want to use high pressure mil spec ammo as well. Thomas M-4’s approach would be more prudent for this group.
Kiz, if your intent is to provide a useful tip, please clarify your OP. I read it the first time and thought, “WTF is he talking about?” I would bet that the DD would have run fine after being broken in & without drilling out the port.
