As seen in Episode 4, the MCRN Marines carry short, bullpup rifles:
Once again, these are modified, off-the-shelf airsoft guns:
The APS UAR 501:
They have been modified and re-painted. A sighting device has been added, the top rail covered, a second barrel mounted above the original, the side rails removed, a fore-grip mounted over the bottom rail, and the magazine removed and replaced with a red ‘brace’ linking the mag well to the base of the pistol grip:
The two above images are heavily retouched photos from the SDCC display. They are from GeekWorldRadio.
Promo stills from episode seven (cropped, retouched, etc):