I got a Bushmaster flat top upper with a 14.5” barrel at a gunshow about 1 month ago, at the show I checked headspace using my Busmaster bolt and carrier from my factory original carbine with a .223 GO gage and a .223 Field gage. I realize there is a little bit of difference between 5.56 and .223 but I was assured that it wasn’t enough to matter for checking headspace. Maybe that was my first mistake. I put it on my Bushmaster lower and shot it this weekend and it worked fine for approximately 100 rds, then it started having short recoil where the bolt would only go about 2/3 the way back and start pushing a shell into the chamber, but since it wasn’t hitting the back of the shell like it should it would gouge the side of the shell with the lugs and push it up enough to jam.
I went home and inspected things and realized the plastic cap on my buffer was coming off so I fixed that and figured that was the problem. After cleaning the gun I went off to shoot it again. It shot fine at first and when I went to unload a live round for a ceasefire I realized I couldn’t get the bolt back without banging the butt on the bench while pulling the charging handle. Tried some more rounds with some sticking and others not, and proceeded to shoot, I was at least happy that I thought I fixed the jamming problem by fixing the buffer, but then it started again, sometimes only ejecting the shell and not even going back far enough to even jam a shell, sometimes loading one, but apparently not closing the bolt all the way because when I pulled the trigger and nothing went off and then seeing the primer was barely dented, and other times jamming with the bolt apparently not going all the way back. The order went
1. not closing all the way,
2. 2. partially loading causing a jam seemingly from short recoil,
3. 3 ejecting spent cartridge but not having the bolt go back far enough to even strip a round at all.
So now I was a little angry. A guy there with a Colt said to spray the heck out of the bolt and carrier with breakfree and try it out. I did and now it worked fine for my remaining 100 rds. Now this may have worked, but I never have to lube my original upper on my original Bushmaster upper like that. I have owned AR’s from Colt, Olympic, and my original factory Bushmaster and never had this problem.
What could be the problem?
My guess is tight headspace causing bolt to bind, thus expending much of the force getting the bolt to un lock and then not having enough to push bolt and carrier far enough back to strip the round. The lube slicked thing up just enough to cause it to work until it got to dirty as before.
1. I cleaned gas tube and gun as usual
2. Used 3D ammo which has worked in all other .223 AR’s including this one with original upper.
3. Tried several different mags, all the same result
4. roll pin for gas tube in front sight installed, however gas tube does slightly move inside receiver with finger pressure, but it doesn’t appear to rub on carrier key. Original upper gas tube doesn’t move at all with finger pressure. Is there another pin to look for?
5. All parts factory Bushmaster.
Anybody know please help me L