Which BUIS allows for the rear-most mounting of an ACOG?

[Diamondback]

I put the ACOG (without BUIS) in the rear-most position where the mount didn't overhang the charging handle. For me it was PERFECT for eye relief! Gonna just mount a BUIS in front of the ACOG.

Going Canadian C7/C8-style? They have the Elcan C79/M134 mount purpose-cut so it'll nest in behind a fixed BUIS at extreme front of the receiver as "standard." (Our C8 is rather different based on a Canadian friend's private-purchase setup for his tour in the Rockpile.)