Richard,
I will be mounting an airsoft M240B on the beast. Inokatsu makes a steel body kit in steel and aluminium. The steel version weighs about 24lbs, the aluminum 16lbs. I can't buy a demilled M240 for less than $4000. And by time I try to buy demilled parts, repair, fabricate, etc, I'll have into it the same an airsoft costs.
The C6 body kit has room inside if I ever want to run a propane/oxygen firing system. Same for the C9A1.
Ideally, I'd like to have the C9A1 for the co-driver and a C6 on the rollbar. But, with the almost $2000 for an airsoft, its cheaper to put an M2 .50 in an M23 mount.
I've got some ideas going for the simplified German-style M2 pintle mount, cast in 7071, heat treated to T6 then hardcoat anodized.