The MCRN carry pistols, described thusly by signage at SDCC:
Designed to fire both self-propelled rounds and traditional cartridge rounds, the rear of the barrel is vented at the back to eliminate recoil for use in zero-g situations.
Retouched screencaps through Episode 6, Rock Bottom, and the ID of the base gun, after the jump:
The base gun is again a WE airsoft gun: the WE X Series IPSC Version. That is a variant of the WE X (Standard) Tactical Pistol, which is a knock-off of the Tokyo Marui XDM-40, a licensed airsoft version of the Springfield XD(M):
Note that many modifications have been made.