Interestingly, this 'gap' that cjt50 is referring to that generally shows with a Magpul beaver tail grip, is the exact same gap that the Mo grip gives when mounted on my Colt LE6290. I installed a Hogue on another AR and noticed the beaver tail touching before the rest of the grip. Installed it on my Colt LE6920 and that's all it took to have no gap.
Odd that there would be different curve on mil spec lower (talking about the AR15, not the polymer 15-22). The standard Moe fits my M&P15A perfectly, but gives this gap when installed on my 15-22 (as is the way my 15-22 MOE is from the factory)