The bolt carrier key screws are supposed to be staked to keep them from working lose. At least that's what I've heard, and the carrier key screws on both my 10s are staked.
An AR can have some play.
Try dry cycling a mag of say 20 rounds and make sure to let the bolt fly after you pull the charging handle back.
If you ride the handle back the bolts rarely pick of the round and chamber correctly.
Once you have verified it will dry cycle correctly take it to the range and let'er rip.
Before you go make sure to pollute the BCG and Rails with a heavy non-chlorinated lubricant grease.
Thats important because it has been determined that those kinds of lubes cause future failure of the metals under stress in the form of cracks,
Mil-STD lubes address this issue.
I used lubriplate on my M-14s and heavier weapons as an armorer and got away with CLP on the lighter pieces such as M16s.
M2s and M60s and M14s and 870s all benefitted from greases on rails and essential points subject to grinding.
Your AR-10 should show some very shiney spots inside.
Those are you essential lube points.
All content of this site is copyright 2003 - 2017 AR-10(T)™, AR-10™, are trademarks of ArmaLite, Inc.® AR10T.com is NOT endorsed or affiliated with ArmaLite, Inc.®
About AR10T.com
AR-10(T) is a community focused on rifles, optics, scopes, gear, accessories, and components used by the professional operator and skilled marksman. Enthusiasts, shooters, and gunsmiths alike contribute to our gallery, articles, and reviews Thank you for visiting!