Cottonwood and Aspen are both low specific gravity hardwoods. Some are definitely harder than others.
I expect those that grow in dryer areas, grow more slowly and thus are harder. Should be heavy compaction in
the trunks of larger trees 4'-5' in diameter, however there is usually a lot of heart rot & conk (rot) branch stubs.
Interesting rifle, Herb, indeed, very interesting.
Elm makes good bows, but perhaps there is a reason it is not used a lot, for gun stocks - kinda like Hickory?