Here is dynno97's MKB42. I give him 90% credit because i added a semi auto mech.
Step 1: Stock
Here is the stock. Build it.
Step 2: Handle
Here is the handle. Build it.
Step 3: Fake Mag
Here is the fake mag. Build it.
Step 4: Semi-Auto Mechanism
Here is the semi auto mechanism. Build it.
Step 5: Fake Barrel
Here is the fake barrel. Build it.
Step 6: Assembly
Here we assemble the gun.
Step 7: Final Notes
load the gun as you would a normal rbg. Play with is sensibly and don't kill anyone.