There is one question I would have to ask you regarding the CMMG .308 lower first, is it the MK-3 (older), or MK-8 (newer) model?
MK-3 looks like this
if you have the MK-3 (their older model) it does have some issues, but I am pleased with it overall.
1ST ISSUE Ė
the front take down pin has just a hair to much material in front of it to allow the upper to hinged like an AR-15, it has to come up. here is my conversation with Tech support:
I ended up getting it taken care of somewhere else, but they were helpful.
2ND ISSUE Ė
my magazine would fall out randomly (before I had an upper receiver) I tried a couple of thing which both worked. The first was to put a very small shim between the right side of the bullet button and the bullet well (in the front of it if youíre looking down range)which made the mag catch stay in. the second solution came from my friend had a Raddlock gen-x bullet button and was bummed because he couldnít use the mag release tool that Phase 5 Tactical makes
so he traded me for my regular Prince bullet button. If you donít install the rubber gaskets that it comes with, and tighten it down, the Raddlock doesnít have the same issue.
And, for the MK-3, as far as I know, any DPMS style upper will work (Armalite, and Rock Rivers will not) I built a SASS upper and I love it.
The last issue you mentioned was problems with the bolt release/catch? I havenít had any issues, but I would recommend the Phase 5 Tactical Extended Bolt release
if youíre looking for one. Hereís a picture of mine.
MK-8 looks like this:
if you have the MK-8, Iím not much help, but after talking to CMMG about my MK-3 issues, they mentioned the MK-8 was the replacement model for the MK-8 but took a different magazine but thatís about all I know.