I have a sendra arms M-16 lower which always worked well with 5.56 for 30 years about 4 years ago I bought FM-9 belt fed upper receiver it has never been able to fire more then one round. What it looks like to me is a feed problem where the belt binds in the feed tray. I lock the side back open the top cover line up a cartridge with the barrel so the bolt can strip a round when it closes. Close the top cover and let the bolt go and I'm able to fire that one round. When the bolt retracts the belt binds and does not align the next round.
I'm not even able to feed the belt manually to me I should be able to cycle this thing manually. I shipped the upper to FO a few months ago it had a broken firing pin they repaired it sent me a video of it working with their FA lower. They are saying the issue could be my lower buffer tube hole is was not machined centered correctly. That could be the case it's a Sendra arms was never a high end lower, but even if I remove the stock and buffer tube I'm still not able to advance the belt correctly it still binding. I can't see how it's the buffer tube issue I should be able to manually feed rounds.
With the lid open and feed tray also open when I put the belt in there move the feed paws manually with my finger it seems like the tray area is too small for the links I have. I know they have a Generation 2 now mine is a Generation 1
My question is if you have one of these are you able to manually feed rounds with the charging handle and how much resistance is there. Maybe this is normal not able to manually cycle rounds from the belt while firing it the force of the blow back is able to cycle it.
FO wanted me to ship my registered lower to them, but I rather not do that if I can figure out another way to resolve this.