The Greener GP = general purpose shotgun was a sporting or farmer’s gun, offered until the 1970s. Though the action resembles the earlier Police – Guns, it was meant for civilian use. I once built a .45-70 rifle on a Mk1 police gun action and a Numrich barrel blank. The Mk1 had a conventional striker, without the later prongs, and a solid, unsplit receiver. The takedown GP receiver is split from below and clamps around the barrel threads by that screw with lever on the left side. I would not trust it to take higher pressure rifle loads.