Compact like this?
20 inches, 6 pounds and all metal except the PVC barrel, auto recharging via remote hose and HPA tank. 190 fps Nerf. Gets an aimed shot off every 4-6 seconds. Hangs on your side. You can crawl with it and hold your own in a firefight with your marker.
Close up of the plumbing and hardware. You can see where the HPA remote hose goes direct into the reg.
Then there is this goodie invented by Fastlearner:
Very compact and can generate a 230 fps nerf in that incredibly short barrel. Uses that plumbing Union fitting as a burst disk type valve. The number of sheets of household aluminum foil controls the velocity. Just activates the 12 gram, it builds pressure and Boom! My tank is dead...