I have never owned a bushmaster 308, so I dont know how heavy the bbl is, but I did have a DPMS which weighed a ton. It was a bench rifle, not a combat gun, so thats what I'm picturing with yours. Now, if your main purpose is accuracy and shooting from a bipod/rest , then there is no reason to go with an acog: on all my heavy AR set ups I have anywhere from a 4-14 or 6-18 scope which (on an accurate rifle) will zap golfballs at 300yds (bipod or benched) . a low-mag scope will not deliver this kind of groundhog-head accuracy at long range. Furthermore, if you do want the CQ option with this set up, just get an angle mount and add a deltapoint dot or the doktor/burris equivalent (you tilt the rifle to the left and there it is) .
from personal experience, I'll never buy another pepr mount. on my 6.8 it constantly shot loose, and had to be tightened to the point that I thought the bolts were about to shear off, and burris never responded to any requests for advise. a heavier caliber would make it worse. But I do agree that good one-piece mounts are inherently more stable then multiple spacers and risers sandwiched together