First Build - 80% lower and unknown upper = short-stroking "frankenrifle"
I Purchased and completed the 20% necessary to build 2 lowers per the rules and regs (Disclaimer). Purchased a used flattop 20" Bull .223 upper, brand name unknown (no markings) but appears to be in good working order. Assembled all and went to the range with store bought, name brand, brass cased amno. Immediately, it started to "short stroke" (extract and eject but not load next round) any where from the 2nd to the 5th round, usually the 3rd for me. My son, who is barely a buck twenty will experience this on almost every round. I attribute the difference to him being much lighter and not leaning in to the gun. I contacted a friend and put my lower on his upper and visa versa every combination functioned properly. I assembled my second lower with a new kit and it did the exact same thing also worked when swapping out with friends equipment. There is a little slop between my upper and lower so installed accu-wedge, nothing changed. I have also changed the buffer spring, tube and stock (no change), I have checked the bolt which seems ok but again this works on other equipment. I have had a few people look at this but no one has resolved the issue. The only thing I can come up with is, I must be just on the outside the specs when combining these two units. I am sorry for the book but I thought it was all important information.
could be the bolt is rubbing the hammer a little to hard on your lower. or the gas key or gas block is not just rite
Being that is seems that the upper will run on other lowers, and it just this upper on your home produced lower the problem, and the fact that you state the upper is loose on the lower, then here are the areas to check.
1. Double check the recoil spring being used to confirm it correct, and that the carrier can enter the Receiver extension tube without binding. Could be with the wedge in place, is cam'g the back of the upper so far that it causing carrier binding in the tube as the rifle cycles.
2. While you are checking the carrier (bolt removed), insert it into the upper alone to double check the key to gas tube alignment.
3. Take a good look at the tail of the disco and hammer for hammer wedging. Could be that you have the FCG pin channels a tad off, and the tail of the hammer is getting wedged between the bottom of the carrier and the trigger/disco tails. If you find white lines/dents on the trigger and disco tail, then you will need to remove material from it. If you find that the bind is to the top of the disco at the hook to the hammer, then you are going to need to mod the disco top to solve that one.
4, Double check to make sure that the mags are not being retained too high in the mag well, and the top of the mag feed lips are not bottoming out on the bottom of the carrier to bind the carrier stoking there.
I will start there and get back with with an update. I appreciate the help as I am in over my head.
First, make sure the upper runs 100%. Then you can eliminate it as a variable.