I have an AR15 (Colt) that would shoot to one POI when feeding rds from the left side of the mag and another POI when feeding from the right side. Someone on another forum suggested that I remove the ejector and round off the sharp edges of the ejector face. This did indeed help a lot with my problem and I now do this to all of my ejectors if they are not already rounded. The theory is that when feeding from the right side of the mag, the ejector's sharp edges were providing more resistance when the case head slides up the bolt face during feeding. This would result in the BCG settling in a slightly different position relative to the round and chamber. Allegedly. True or not, the problem pretty much went away when I rounded the ejector.
I round mine by chucking the ejector into a drill and holding the ejector at an angle against an old "carborundum" sharpening stone while spinning it with the drill. All it takes is to round off the sharp edge.
Since that time (early 2000's) I've seen pictures of high end bolts with with the ejector face being completely hemispherical.
I don’t put a bolt together without doing this for larger case head cartridges.
The Dutch started doing it with their AR-10 production way back in the late 1950s-1960s, and those rifles tend to be freakin’ accurate, unbelievably so in the samples I have shot.
I mainly got into that technique from Hi-Power shooters who were shooting 6mm PPC, then 6mm AR, using Colt 7.62x39 and later Grendel bolts when Grendel came along in the early 2000s.
The reason I do it is for reliable right side cartridge presentation, and I never though about it contributing to consistent chambering and bolt lock-up. Could be another contributing factor into why I consistently see such good results with pretty much every barrel I have built-up around.
45-90 also points out that he uses tunable alignment bearings on his bolt carriers, so the tail can’t get out-of-center at any time during the cycling of the action.
Les Baer and someone else used to make Match carriers with oversized rails that ride inside the carrier raceway tighter, though that can degrade reliability really quick as the gun fouls.
There’s also a buffer with a rounded alignment dome that sits in the center of the back of your carrier to keep it aligned.
Squaring the receiver face and uses high-end barrel extensions make a difference.
When you add all these little techniques up, it can start shaving off fractions of an inch in average group sizes.
I feel it’s best to start from the get-go with as many things setting you up for success if you’re truly chasing bug hole groups.
I wish AA still made that tri-lobal firing pin that doesn’t drag as much inside the firing pin channel.