For you budget builders:
Old ARMs 22m68 with cantilever mount, compM version milled on to allow newer m2/m3 aimpoints to fit. This one fit my Colt & LMT uppers w/o slop. Used overseas years ago. $20
Command Arms M33 two rail 6 & 12, m4 handguards, new $45
http://www.commandarms.com/product.asp?pID=8&cID=8
Both $60