will a subpar bcg affect accuracy? If so how? I dont see why it should.

It shouldn't as long as it doesn't make your rifle go kaboom... Also, as long as it keeps cycling.



People worry about a quality BCG because it's one of the primary points of failure (specifically the ejector or bolt itself) of the lower quality manufacturers.

No.

A subpar BCG will fail or wear prematurely. It will affect cycling more than accuracy. A kaboom caused by premature BCG failure has been known to cause extreme excretory malfunction.

Slight variations in machining on the bolt may cause a different lockup than another?

I'm not an expert though....

Well, in bolt rifles the consistant lock up, and making sure all surfaces on the bolt and reciever are at the correct angle makes all the difference.  I assumed that is why semi autos don't seem to be quite as accurate. Most semi's are either military guns, or hunting guns.  However there are no semi autos made specifically for precision shooting so we are not really comparing apples to apples.   I have some very accurate AR's, and and accurate H&K and FN FAL.  I guess the way they are built is what makes the difference.   A bcg that has a lot of play or inconsistency could possibly cause less accuracy.