An often overlooked inspection point in this platform is the oprod pocket/bolt roller contact points
The very rear inside face of the pocket makes contact ,as it should, with the roller while it manipulates the bolt into lock up.
What I have found both in inspecting and building these rifles (m1a's, LRB's included) is that some have contact between the rear inner face of the oprod and the roller, at lockup. So much so that the oprod is putting forward pressure on the roller with bolt in full lockup.
This is very bad and a roller destruction is imminent. There should no forward pressure applied on the bolt roller, by the oprod , at full lock up. The pocket retains it in lockup, yes, but there is a teansy bit of wiggle room that should be there.
If a guy has minimal to medium experience with these rifles, inspecting and remedying this problem may be better suited to experienced hands.
If the above is not the reason, I dunno.
Bolt rollers hitting receivers , "under" the roller at lockup, is a problem known to plague production runs of springfield armoury inc M1A rifles. I've never seen the issue documented on norincos/polys.
Spitting a roller on an otherwise new rifle, being it's chinese, I would look at the oprod and whether it's pressing forward on the roller in lockup. If it is, the rear face of the oprod pocket may need a few thou" or so removed to clearance
To test a given rifle, clean all grease/oil from action parts. Reassemble
Cycle action, letting oprod violently slam bolt into lock up as you would if you were loadin her up to shoot.
You will need an instrument to poke at the roller with to see if it will spin. If it is relatively eaasy to get some spin without feeling drag from the rearmost face in the oprod pocket.... Yer good to go.
If the roller is being jammed by the oprod pocket.... Well you see the point of my post right here
You do not want to get carried away removing material, "just' enough to free up pressure contact.
