bipod mounting brackets opening 2 3/16" typical both.
left and right axis drift is easy to see with a string line down the top of the gun . start in between the front sight ears to a small mark in the center of the rear buffer mount area. don't trust that the rivets on top for the buffer lugs are exactly centered.
up and down axis drift is harder to gauge but the same string line method is still the best IMHO.
also if you make a perfectly vertical line on a wall and carry it out slightly on the floor you'll be able to set the receiver down upright on its buffer area , check with a level sitting on top if its square ( if the floor is ), and stand back and look to see if its plumb ,turn it 90 degrees to check both axis.

submitted by 7.62x51
