I have Academy's M12 155mm self-propelled cannon, but was thinking of replacing the existing gun barrel with an aftermarket one. Barrel Depot makes one of course, but unfortunately it doesn't have the bottom weight or the sliding bars molded on. Is ther another aftermarket M12 barrel available, and if so from who? Thank you.

Azimut makes what you want, along with photo etch. It's a question of how much you want to spend. Personally, I find that the kit barrel is fine. If you don't have a Flex-i-FIle, I'd recommend one. The kit has other deficiencies which I find need more attention than the gun assembly.