But, to make it shoot, is it better to get the length of the bullet close or the weight, for the same shape?
I think much of it depends on the rifle and what you are trying to accomplish. I take it this is a double rifle so it may likely have been made to shoot more than one bullet weight. The rate of twist must be fast enough to stabilize the bullet, for hunting accuracy a bit faster twist will not be an issue. Many other factors go in to getting a black powder double to regulate such as powder charge, wadding, bullet lube etc.
I would be interested in what the mass spec machine shows.
