stevenb,
Someone on AR15.com bought & evaluated the B&T rail, & it didn't deliver. Among other things, gas fouling blows out between the vents on lower end of the rail & onto the shooter. His bottom line: stay away.
I'm curious to see what SF will come up with, & I hope it's better than their AR rail, which doesn't line up w/ the receiver picatinny flat top. This Sidearmor unit looks nice, but I still have to see them all before I'll commit.
BTW - what M4 "fix" are you referring to?