Hello all, I appreciate you taking the time to read my post.  I am currently in the process of building an m4gery for work (colt leo lower pre assembled trigger group, 14.5 colt 1/7 barrel w /phantom flash, A3 upper , sir 50, eotech 551, m900ab, colt 6 pos stock, and 4 rail dpms gas block w bushmaster flip up front sight.   Well I have all the listed parts but ran in to a slight snag.  I have built 3 ars to date but none with the 4 rail gas block and little did I know that the the Front handguard plate, that is usually mounted directly behind the gas block was not meant to be used.  As a result I mounted the plate gas block and phantom and sent it away to kkf to get it pinned.   As I first shot I Noticed numerous malfunctions and checked the gas block to find that in fact the gas block barrel hole was within the carbon ring on the barrel formed by the block, but barely.  To remedy the situation I taped up the barrel to protect it and filed the sh*t out of my gas block to help center the barrel gas hole in the gas block.   After I shot it my, gun  continues to malfuntion even though I am about .02 inch off the perfect center of alignment, but well within the observed carbon ring left on the barrel by the gas block after firing.  By malfunction I mean that it sometimes does not pick up a round out of the mag after firing(then goes over the round instead of seating it) and the bolt will not hold back after the last round fired.  To me this means gas problems still.  TO double check I threw a differnt upper on my lower and tried the same ammo and mags.  THe gun performed fine with the same mags and ammo, so I am saying that the problem lies in the upper.  Any way after my novel my question is what little ******* thing am i missing that may cause the problem I have described.  Any advice will be helpful thanks, kenner

Just a guess, while waiting for someone with more knowledge to respond, but could it be that the gas hole is undersized?