John Garand brought a rifle to Aberdeen proving grounds to compete against the Petterson rifle. The Army likes the Garand but told him nice gun but wrong caliber it was chambered in 270. He was informed a war was coming and all the ammo world wide was 30-06. He went back to Springfield Armory and returned with a rifle chambered fo 30 cal. It was modular. The garand trigger group built at Winchester works in one from Springfield rifles etc. I have one but it’s not my SHTF rifle or the one I would carry into battle today. I have better fits for current combat conditions. The great thing about the garand rifle was it’s ability to work in a lot of environments reliably. But taking a trigger group from one weapon to another has some advantages in the field.
Hey I get it. Its not the idea that’s bad. Whether it works or not is all in the execution of the idea.
BTW its been a loooong time since I have field stripped my Garand. (I enjoy it more as a conversation piece than actually shooting it). But I don’t recall in field stripping it the barrel coming off the receiver. What I recall is the trigger group coming out as a unit. Maybe its semantics as to what is defined as the receiver. But in the Garand’s case, I don’t see how the basic alignment of the shot can change (chamber/barrel/sights), which does not seem to be the case with the P320 design.