[BMT (RIP)]

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The gunsmith screwed up the end of the barrel when he chopped it -- severe damage to the crown or the end of the barrel was cut off center -- thus having one side longer than the other. This would also cause bullets to yaw and pitch wildly as gas will escape from one side of the barrel before it escapes from the other side when the bullet leaves the muzzle. Remove the flash suppressor and check the crown and check to see that the end of the barrel was cut square. You should be able to see that one using a magnifying glass.

The flash suppressor was poorly attached to the barrel. If so, you should see damage to one side of the inside of the flash suppressor where the bullets were hitting it as they were fired. If the gunsmith threaded the end of the barrel for the flash suppressor -- he may have threaded it in a manner that put the flash suppressor off center. If he used pins -- chances are the holes he drilled for the pins damaged the barrel.

The flash suppressor is loose or was made out of specs. Thus the bullet is hitting the side of the flash suppressor periodically.

Your buddy is using a 75 or heavier bullet in a barrel twisted one turn in twelve (55 grain and lighter bullets). Check the bullet weight even if he swears they are 55 grains or under.

Your buddy is using a powder that is way, way to slow in burn rate for the bullet he is firing.

Remove the flash suppressor and see if that solves the problem. If so -- flash suppressor issue. If not, the end of the barrel isn't square or the crown is severely damaged. Either can be fixed.