George,
the M-41A is available as a resin model, do a search on eBay and you'll find a few of them. They were made by SHED Models in England, as well as ICONS in the US. Current price for the "stunt" resin kit with functional LED counter is in the $600 range, there's a cheaper one without the counter available. FWIW, here's what I know about the mentioned guns:
M-41A:
Thompson M1A1 mounted in a resin shell, with a Franchi SPAS-12 trimmed down and mounted under the Thompson as the "grenade launcher".
"Smart gun" (Vasquez and Drake):
German Rheinmetall MG42/MG3 7.62mm machine gun mounted in a "steadicam" stabilizer rig. They used the 50-round belt drums for the MG, and in one scene you can clearly see the non-fragmenting empty belt sticking out of Drake's gun as he blasts away.
Tamara,
Robocop's sidearm:
Beretta 93R with a huge front end which doubled the pistol's length. It had a long, ventilated barrel and a massive square barrel weight. In the scenes where Robocop fires the gun, you can clearly see the Beretta slide go back, while the rest of the arrangement stays in place. He also uses the gun exclusively in its three-round burst mode.