CMMG works great. The Geissele is *really* good and it only requires a minor modification to the bottom rail to work though. If I had it my way, I'd pin the Geissele block on and trim the interior of the bottom rail so it fits.
Pinning it isn't an option. It's a V7 barrel and the groove for the gas block roll pin comes drilled from the factory. I had originally ordered a pin on titanium gas block from V7. Their gas block doesn't come completely drilled out. Instead, it comes with a starter hole. I fucked up the block trying to drill it out, which was completely my fault. Anyways, I got frustrated and ordered a block from Bill thinking that'd be the safe solution. The hole on his block (which does come completely drilled) is not only a different size, but in a slightly different location.
I just went back out and decided to take a second look. I got the lower half of the rail on by squeezing the muzzle and upper half together. Not ideal, but not the end of the world. It was hitting the heat shield.