We QC all the parts. Each rifle gets at least 5 rounds. If anything on the rifle seems fishy, we put 20-30 rds through it. We use the low-pressure wolf. We figure if it won't shoot the first few rounds through that, we're not gonna send it out until it's right.
For the most part... the problems that escape our scrutiny are erratic ones. A bolt puncturing primers, for example, is usually erratic. They will punch through a primer every six rounds or so.
A poor gas block will sometimes work, and other times can cause stove piped rounds and failure to eject.
And so on. And really, it's human or machine error when there is a problem. We do our best to make sure none slip out the door, but sometimes things happen. We always learn from our mistakes. For example, we are now very, very careful with gas blocks and bolts.
*** We have ways of testing the chambers of the barrels to make sure there will be no accuracy problems. There has only been one issue that I know of with poor accuracy. That barrel is in the shop now. Keep in mind, it is 1) a light barrel and 2) has quite a few rounds through it. The guy bought it used, but I offered to check it out anyway.