Re: Mag choices for 308 ARs
I have owned and still own several rifles compatible with the HK G3/91 magazines. The are very reliable and well-made mags and they function 100% out of the HK rifles. I can't say how well they'll function in an AR-platform rifle. As others have said, you do lose the bolt hold open (BHO) on the last round with the G3 setup in the CMMG lower. I understand they are working on a solution for that issue, but nothing yet. Also, G3 mags are available in a couple varieties, including the super-cheap aluminum mags that you see for $1.25/mag or so and the steel mags which are much mroe expensive. The Aluminum mags are great for range mags and lighter duty use, but were designed to be lightweight and disposable and thus, won't hold up to long-term use/abuse as well as the steel mags.
If it were my choice to make, I'd go with a DPMS/KAC type lower and run the new 308 Magpul PMAGs that Dustin mentioned. I have been running them in my POF and they are 100% reliable and will allow you to load and feed heavy, 175gr match loads in the mags. They were purpose-built around the M118LR round.