Although I own a couple GM barrels, none of them was in 40 cal. I am nowhere good enough of a shot to give a "valid assessment" of one barrel brand over the other.
I'm not saying I can't hit the side of a barn, but I can't put five shots touching each other at 100 yards with a BP rifle, and because of that it doesn't matter if I'm shooing my GM, Colerain, FCI, Rice or Oregon barreled rifles - "for me" they are all "more accurate" than me, so they are all "equal".
I did shoot a 40 for a few years. It was a 7/8" Colerain and it was my "deer rifle" (in Ontario there is no minimum BP caliber for deer). It was deadly out to 50 yards. I never pushed it beyond that and never intended to either. If I wanted to shoot further I have a 45 and a 50, both of which are better suited to distance.
And yes, a 40 is more than enough for Bambi at 50 yards or less if you don't take "iffy" shots.
I retired the 40 as my deer rifle. I replaced it with a 38, which again, is enough at sub-50 yards.
If you can't use the 40 for deer where you are, it may be suitable for Turkey or Coons/Fox etc - a little large maybe for squirrel but certainly "doable".