AR-50A1
The lower portion off the stock is milled away to allow the Precision Rifle ABR to be installed with only adding the height of the rider. 3 Bolts hold all the parts.
The Rules for adding an ABR
Do not raise the rear stock more than absolutely necessary
Space behind the grip to the front of the ABR minimum is 2+” so the adjustment hand is not hit during recoil
ABR parallel with the bore