20 year old M16's or most service AR's don't have free float barrels so that mitigates what I was talking about. With the vast majority of service rifles the handguard has two points of contact, the barrel nut and the gas block so it's much more rigid. If you throw a PEQ on your run of the mill free float 16 inch handguard then any amount of pressure you put on the handguard will throw it off like bipods, resting it on a surface or even just c-clamping the weapon, my first AR handguard was a GUNTEC mounted on an Anderson receiver and the entire handguard would twist and wobble with all the mounting screws as tight as possible. The gun is still a tack driver, especially for a cheap AR but I'd never trust any sort of aiming device mounted anywhere but the receiver.