I just got one of those uppers on sale and same thing with NFA lower. Crazy thing is I have the taccom barrel on another PCC with NFA lower/ upper and flat nose feeds just fine. I have no idea whats going on, but it frosted me so I stuck it in the back of the safe and Im shooting pistol next match. LOL
Edit: Im not convinced I didnt do something wrong assembling it. I'll look eventually...