LMT has a .071 port for military ammunition. Colt has a .093 port to overgas it and run everything. Either start shooting military ammunition or open your port. What buffer do you have?
i've used standard, h, h2, spikes ts, also tried different bolt carrier assemblies, BCM, Colt. i've tried m855,xm193,remington umc, silver bear,pmc gold, georgia arms, privi 55gr and 62 grain. i do understand that these uppers are intended for military/leo use. i have tried the higher pressure LC ammo and it does run more reliably, but will still short stroke. i try to not to be a "know it all" when i contact a manufacturer, i actualy tried to explain the situation i was having and that my pistol is having issues. LMT has never heard of an AR15 pistol according to Gene, i explained that i purchased Spikes pistol tube kit which is the same a "mil spec" buffer tube and uses the same carbine spring and buffer as a M4 or any other AR15, it just wont except a butt stock for legal reasons. i had a friend that has a reistered SBR use my upper on his lower assembly, i was trying to rule out "limp wristing" or some other issue a AR15 pistol could possibly cause. i was asked if i was military or LEO, i answed no and the "high hatting" started.(LMT) "why do you have a 10.5 upper?" don't you know that these are designed for the 77 gr bullets for the military/LEO's". after a few minutes of expalining that i've used different lowers, buffers, ammo and that a Colt and Spikes uppers would shoot the ammos i had, he then offered to look at it, if i paid shipping. i can pay shipping to someone that doesn't act like an arrogant douchebag, and to someone that appreciates my business. i'm an avid shooter and love this hobby, i spend a ton of money on high end parts and gear, so i can have weapons/gear that will last and that i can hand down to my boys, sorry for the mini rant