Hi Kynoch,
Interesting gun you have there, petty light at 13lbs, I would guess that it was not meant as a 10 Dram gun, that load if black powder in a 13lb gun would really rock your world.

Curious to know who the maker was, what is the barrel length, and the chamber length.

My G.E. Lewis 8 Bore rifle is 15 1/2 lbs and is a real thumper on the shoulder with a full 10 Dram load. Most 8 Bores for the 8-10 Dram loads seem to have been 15-18lbs.
About 30 years ago I had a Greener 8 Bore "Cape Gun" (right tube smooth, left tube fully rifled) that weighed 13 lbs. It did take 10 Drams to bring the rights/lefts together with round balls......& it was a real kicker.

The beauty of your gun is that if it shoots nice groups at 6 Dram equivalent of Blue Dot then you are well set for fun shooting without too much of a beating.
- Mike