Actually I heard a gas MG-42 was made a few years ago. But if your going to use a gun other than an MG-42, the M-60 would probably be a good gun to use. Well cosmetically speaking. I don't know how reliable it is (it's gravity fed.), but it looks very similar to it and the real steel M-60 was based off of the MG-42. Either way, I can't wait to see how it turns out. Good luck.
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M-40 grenade: $30
Pulse Rifle: $2000
Tactical Smart Missles: $10,000
Sulaco Full of Badass Colonial Marines: $700,000,000
Kicking some alien ass: Priceless