I actually want to play up the Asura aspect and have the gunslinger be a meditant teaching a warped version of buddhist principles (Life is suffering, seek asceticism/detachment/destruction/emptiness) and have the cannon be a gigantic metal pillar/prayer wheel with sutras of creation and destruction embossed and gilded over it's body.
It's really really over the top but I personally think in a very cool way.
If you can't lift it, you can't say you're wielding it 2handed, can you? Obviously we need mithral. Or possibly antigravity metal.Google+Wikipedia can't get an exact amount (it varies by size and period), but 25 pound cannon balls says larger than the 4800 pound "culverin extraordinary" which fired 20 pound balls, so yeah, far, far outside your limit. Such a cannon would be on wheels (you can push/drag 5x your weight, and I think having wheels would boost that, but I can't find such), but then you are boned if you have to go up stairs, like in a dungeon.