The MAC ram is an anti-chop ram! That was why it was first introduced. The first of it's kind for Bushmasters. The adjustable position was just a secondary bonus to the design. The GBR works on the same principal as the MAC ram only without any adjustments. The MAC is adjustable via setscrew on the back and the internal position insert. I don't know much about it but I'm sure others here or on www.icd-owners.com
have played with them and do.