Can anyone ID this part? It appears to be a CO2 BB gun magazine for a pistol. It's a heavy die casting, the magazine assy. also houses the CO2 cartridge. There is a threaded cap in the base that reveals the compartment for the CO2 cartridge. On one side, it's marked, ".172" CALIBER-4.38mm". It measures approx. 5-1/2inches long. Thanks.