Re: ar10 lower that accepts hk93 mags
LRBHO = Last Round Bolt Hold Open (the bolt will not lock back on an empty mag...manual lock-back ONLY)
Once you get the upper milled out, they run pretty smooth. I helped a friend of mine get one up and running last year. The modifications to the upper aren't extensive, but having the right equipment helps. You might get away with a "dremel" solution for the upper, but it will likely look like crap and could mess with how well you get it running. Use a 1/8" end mill like CMMG suggests and you'll be in and done in no time with a clean setup. Remember that you are going to chew up the annodizing on the upper, so be prepared for a refinish job too when all is said and done.
The super-cheap G3 mags that you see are aluminum ("throw away") mags, so they don't take a beating like the HK steel mags do (HK designed them that way), but they work well while they are in good shape (at $2/mag, when they do get beat up, pitch 'em and start over). As for the LRBHO issue, last I heard was that CMMG was working on a solution, which I assume will involve modifying the followers and probably the back of the mags so that they actuate the bolt catch (my guess...I don't know what specifically they have in the works). When I called to talk to them about the mods needed to my friend's upper, they were very helpful and glad to give me some tips on the modifications, etc.
