Have worked in and around the gun industry a big part of my life. In general, it is not illegal to own or posses such things. As stated above, the frame is usually the serial numbered part of the gun that has to be registered etc. A few exceptions, such as suppressors, etc. are restricted (without the proper permit to posses), but as a rule barrels or internals are just parts, not guns. Just a note; the new Sig P320 goes by the trigger group, and not the frame. You can buy the entire gun, and then additional kits (barrel, frame, and slide) to switch the trigger group between. This allows you to change size, and caliber without the hassle of buying multiple guns to own multiple calibers.