My suggestion would be to use one of the Biakal MH18's the barrel is screwed into the Monoblock so the small bore barrel could be removed and the barrel of your choice fited this was the one in .222 rem I had:-

That's as it came but I didn't like the stock style so I altered it;-

As you can see the barrel screws into the mono block wihtt he lump that locks it closed:-

It has the latch you want.

they are also reasonably priced but would require the trigger worked on.