I recently purchased a Del-ton 20" hbar upper with rifle length gas, they only had them with an A2 front sight so I figured I would just cut that down to fit under a Midwest Industries 15" hand guard (This is mostly a range toy/coyote gun and weight was not a big concern) The upper came test fired and I could verify that with the small amount of carbon residue on the gas key and the end of the gas tube that enters the receiver. Basically as soon as I got it I tore it apart to mount the free float tube. I swapped barrel nuts and cut down the front sight and put it all back together. I took it to the range to sight it in and from round # 1 it was short stroking. It would fire a round but fail to strip off the next round and would slide it forward with the friction on the bottom of the BCG and then jam the round at about a 45 degree angle with the tip of the round in the chamber and the bolt riding up on top of the round scratching it and denting the bottom of the round on the feed ramps.
After I figured out that it was short stroking I tried to narrow down a cause. I am using an A2 length buffer tube and buffer that I took off of an old Bushmaster post-ban carbine and they worked perfectly on it. I tried to replace the buffer and spring with a loaner from an Armalite that was brand new, same result. I started by firing some frangible .223 that the local range requires you to use so you don't put a hole in their roof. I then tried some of my own reloads that work great in my carbine (21.5 grains of Reloader 7 on a CCI small rifle primer with a 55 grain FMJBT) I then tried some of the ranges standard velocity .223 reloads (unknown powder charge and primer but 55g bullet that works great in my carbine also). These were able to get one to three rounds to fire but would eventually jam. I also tried some PMC .223 factory ammo and could get at best 4-5 rounds to fire but usually only 1.
The next thing I did was try different magazines. I didn't expect all these problems so I had a very slim choice. I tried a few 20 round P-mags a 30 round P-mag and a 30 round USGI mag I bought off a friend in the corps. All 3 had the same problem with the jamming and not one of them had the bolt lock open on the last round (this is where I decided short stroking was definitely the problem.
When I got home I tore it apart again and measured every thing to make sure it was in spec, these are my results:
The gas port on the barrel 0.089"
Port on muzzle end of gas rod 0.1245"
Port on the bottom of the gas block 0.144"
The mushroom end of the gas rod OD 0.1795"
The mushroom end of the gas rod ID 0.119"
Gas key ID 0.181" (bolts are very tight, staking seems to be so-so)
There was no evidence of a leak on the gas block on the barrel from residue nor did it appear that there was any gas escaping at the muzzle end of the gas rod.
From what I have found the gas port is in spec so I shouldn't need to enlarge it. Would it be beneficial to remove some of the weights from the buffer? The buffer spring also seems to be in spec and has an uncompressed length of 12 and 5/8 inches.
This thing has really got me stumped and any input would be greatly appreciated.
What are your thoughts on either removing weights from the buffer or swapping from the rifle length buffer spring to a carbine length buffer spring?
The first thing that comes to mind is a gas problem with the A2 front sight that you cut down. I assume it is pinned on so there is no chance of you not having it back on correctly. Still, I would grab some calipers and measure the distance from the back of the gas block to the center line of the port, and measure the distance from the barrel shelf to the center line of the gas port and see if they are in fact lining up correctly. The distance from the shelf to the port should be longer than from the back of the gas block to the port since you are not using the same handguard, so be sure to allow for that. If the gas block is set screwed in place and after going to the float tube the GB is now jammed up against the barrel shelf then you could be blocking part of the gas port, but like I said, if it is pinned on, GB position is not a problem IF it was installed correctly from Del-Ton. Take measurements to verify.
Next, make sure that during your grinding you did not get something down in the gas block that could obstruct the gas flow. Since you ground it down, make sure you did not go too deep and actually open up a small hole in it anywhere between the barrel port and the gas tube. Very unlikely but something to check.
Next, while you have the GB off, check to make sure there is nothing obstructing the gas port in the barrel also.
When you changed the handguard, make sure you did not somehow put a crimp in the gas tube. If you did any extra twisting of the barrel nut while the gas tube was installed to get the rails to line up and put a little too much oomph on it you could have slightly crushed the tube. I don't know if that is even possible with your handguard but something to consider if you did tweak the barrel nut while the gas tube was going through it.
Then move on to the gas key, make sure it did not come loose and it was coincidental that it loosened up at the same time as you changed the hand guard.
If the gas port is clear, the gas block is clear, tube not crimped, key not loose, then it should be locking the bolt back after the mag is empty. I assume you did that test to make sure it is really short stroking and not over gassed. A BCG that moves too fast can give the appearance of not having enough gas when feeding and that may be the case IF it locks back consistently when shooting with an empty mag in the rifle.
If none of these, I'm out of ideas. You should not have to reduce the weights in the buffer or go to a lighter buffer spring. The 5.56 has been pretty much standardized so it should be working with standard buffers and springs if everything else is installed properly.
Two other things. When you put in an empty mag and pull the charging handle to pull the BCG back does it feel differently than when there is no mag in the rifle? If so your mag catch or even lower may be out of spec and letting the mag insert too far into the rifle creating too much drag on the bolt carrier. A high hammer can also cause drag on the bolt carrier so check for hammer drag on it also.
If pulling on the CH feels different when an empty mag is inserted you may have the mag catch problem, if pulling on the CH seems normal at first then gets really hard even without a mag in the rifle it may be hammer drag.
Now, how can one beat that? Kudos Del-Ton.