Had a client shoot a grouse up in a tree, with a .32 TC rifle, using 25gr. 3F, seems to me. That shot opened the grouse up like a book and it was all there, except for the breast meat - that was gone. So- you need to hit them in the head, whether grouse, squirrel or rabbit. A .36, with a bag hit, will open a snowshoe hare up like that as well. Imagine what a .40 will do. I see a .40 as a squirrel rifle, up to coyote/wolf rifle, within reasonable range, that is.
My .40 cal Douglas is from the mid to late 70's and its a 1:66" twist. Shoots one-hole groups with anything I shoot in it.. Doesn't matter 2F or 3, under .395" round ball and tight fitting patch. I use exclusively Dynamite Nobel caps for the most consistent ignition...Bart
Smokey I have three rifles. 32 - 36 -an 40 caliber all flint all Douglas barrels. I shoot lite loads in all of them an some medium loads up to 55-70 grains. All very accurate ..! Never ever had a problem with Douglas barrels..!I like 35 grains in my 32.. It works is all I know..!! What works for me might not work for others. But that's half the fun...Oldtravler