I recently picked up the KWC M712 and a few Tercel magazines (they're on sale on RSOV for $11 a pop) to try in it.
I can't really seem to get it to work. I filed down the guide rail part in the magwell as renegadecow suggests so the Tercel magazine now slides in and out freely. However, I've encountered a couple of problems.
1) The magazine catch doesn't want to engage with the Tercel magazine. Inserting the magazine and lifting the gun up results in the mag falling out. It doesn't matter if I try to slam the mag in or gently push it, same thing either way. If I hold the magazine in position (which seems to be with the little nubs on the side of the magazine approximately 1mm away from actually touching the magwell like they should) then I can fire. Gas efficiency is terrible though. Maybe 8-10 shots on a full charge of propane.
2) The nozzle and bolt seem to get caught on the feed lips/blow off port of the Tercel mag. This is (mostly) remedied by keeping the mag down ever so slightly as mentioned above. I would imagine this is also the thing hampering gas efficiency.
Has anyone had similar problems with getting Tercel brand M712 magazines to work in their gun? What did you do to remedy it if you did? I'd love to use propane in this thing instead of CO2.