!/2 inch gives a lot of possibilities counting for corrosion loss.... It still looks like a break action barrel to me, it's a shame you didn't find any more of it. I think I'd have to go dig around the area and see if any more parts turn up. Don't get me wrong, it's a really cool find whatever the barrel turns out to be and if it's a civilian gun that someone stuck a bayonet on, well to me that just makes it more interesting wondering why they did it.