Hi, I'm just slightly confused, I could take your question two ways:
1. What were typical weights of British 6-pounder cannon so I can mark mine with a typical weight?
2. How do I convert the weight of mine into hundredweights so I can mark it properly?
I'll answer the second one since it is easier, and is the way I'd do it. British and also some early US artillery pieces were marked in the tradition of British commercial castings which were typically sold by weight. Anvils were also marked this way and I have one, and I only bought it because it had the hundredweight marks on it.
Here's an example. Let's say you find a cannon marked under the cascabel with 5-3-11. First digit, 5, is hundredweights, or 5x112 lbs. Second is quartels, so it is 3 x 28 lbs. The third is single pounds so add 11 lbs to the total.
560+84+11 lbs. if I did that right.
I'd recommend weighing your cannon barrel when it is completed, and still bare with no paint, then converting the wt to hundredweights and marking the cannon with that weight. Sorry but I can' t see marking the gun with some "typical" number that isn't the actual weight, unless yours is a scale model or something.