Hello all, New to the AR world just finished my first 2 weeks ago. Stag lower (new) with a used FTF industries upper. . My first time on the Range using M855 ammo was frustrating. First off BCG does not lock back after last round is fired. I manually charged the rifle most of the day trying to get it to cycle. Several times out of the blue it cycled through 30 rds like it should. Some times it would cycle five rounds and fail to feed. The bolt would also slam into round halfway into the chamber occasionally. Leaving gouges in the the cartridge. I have ordered a new gas tube and a mcfarland gas ring, I did come home clean the gas tube blew compressed air through the ports. and that did not help. Basically confused because of the inconsistency. Used several different Mags. 2 new magpul, several of my brothers nato mags that are known to function. any help will be greatly appreciated. Looking forward to your replies. Thanks

Since you are new to AR's and your rifle is new it may simply be a lubrication issue. A lot of folks run there ARe pretty wet. A new rifle will need to be lubed well until broken in. I'm sure others will chime in with more detailed brake in suggestions.

You forgot the most over looked new gun problem. Failure to clean the ever lovin' hell out of it and thoroughly lube it before that first shot.

edit: Forget my goofiness, I just re-read the op and found it's a used upper, just a new lower. Carry on.

Do you have a carbine buffer spring? There is a difference between carbine and rifle buffer springs.
Do you have a carbine buffer? There is a difference between carbine and rifle buffers.
Does your bolt cam properly in the BCG? The bolt rings and cam pin sometimes have to break-in a bit.