My plan for the barrel is the leave it full threaded so it will turn off. I have made an additional lug to attach to the existing receiver recoil lug that will give me more surface area to attach a steel plate to. There will then be a matching steel plate attached to a lug on the barrel.

I am leaning towards using a spring loaded check ball on the plates as a locating point for the barrel. Then a pin driven by a finger latch on the bottom of the for end for actual locking of the barrel. I plan on re-purposing a Browning Superposed latch for this. I like that latch because it clips in firmly, but when unhooked it is free and loose. That way the locking pin will stay retracted for barrel removal.