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.