I took out to brother's house and tried it. the 150gr ammo ran flawlessly as usual. The 125gr ammo seem to run a little better. I only had 1 failure to feed (like in linked pic). The bolt did lock back after last round everytime too, which is an improvement. I couldn't tell what direction the brass was kicking out since I was all by myself, in a ditch, in the woods.
Without knowing the powder used, it’s impossible to rule the ammo out.
There is no reason that a carbine length system should not cycle, even most factory subs without a suppressor. Even more so since the port has been drilled to maximum.
Easiest thing to do is try a different lower since you have one and make sure the buffer is a standard H. Normally a standard buffer is not marked, but the marking can wear off, so you can’t count on that. You can probably feel the difference if you have an H2 to compare.
Gas leaks or restrictions, binding bolt carrier, heavy or incorrectly installed hammer spring are all things that can slow down a bolt and cause feeding issues.
For comparison my 16” carbine is set up with a .097” port, full auto carrier, H2 buffer. The buffer is heavier, port smaller than should be and I still shoot subs in suppressed. The key is not letting the carbon build up and good lubrication. It’s a target rifle, not combat. I don’t need it to function at -40 and full of dirt on full auto.
To have a rifle not function as yours is, there is a serious problem with the ammo or gun. Maybe both.
Based on the video I linked for you, we already know the ammo is suspect.
I might buy, beg, borrow, or steal some known good subs and see if they function better than the supers, the problem is obvious if they do.
