Hobbnob: First Person Airsoft features a MK12 Gas Blowback Rifle Build in this video... "This is a gas blowback MK12 Mod 0 SPR based off the Tokyo Marui MWS platform, and the story of how I made it and how it's turned out.

It's worth noting on the muzzle strikes when firing quickly that it's possibly not muzzle strikes but bolt/nozzle/FCG interface issue, potentially caused either by the Nova lower not quite marrying up with the TM upper, or a bolt bounce issue. It's not a big enough issue for me to be concerned about, but it's a plan for future research."