SMR is an over-used and under-defined term (probably why it is used so much).
In Tennessee Rifles the most common calibers were mid-thirties to low forties (remember, there were no such standard calibers BITD). But there were also many smaller and some larger. In Tennessee originals there's always an exception somewhere, but mine own 54 is quite a bit "oversized" for that general type of gun. I was headstrong and certain at that time and it worked out for me, but I wouldn't do it again. 45 would be much more appropriate-just as 42 or 38 might be.
As to Daryl's admonition about it being small, he is way up north where animals tend to be larger (heat conservation) but here in the South where animals are often smaller (heat dissipation). I'm guessing a fat doe up there might be 200# and that's a real good buck in my part of the hills. I've heard of deer much larger from the Northern Plains states. The one doe I've seen shot with a 40 was killed decisively and quickly with a body shot (specified because I have another pal who takes nothing but head shots).
So where you hunt and what you hunt and how well you handle the stress of hunting and shot placement might be the best factors as to caliber selection. And please never consult ballistic or power or killing tables constructed by modern ballisticians. I used to bleed external ballistics tables, but (after much study) found them less relevant in the BP world. Roundballs simply kill better than paper can ever explain, provided they are put where the need to go.
I'd have zero problems hunting in the South with a 45, but I'd want more lead if I was in moose and bear country-as D is. Game laws notwithstanding. My state allows 36 for big game (which I think fits the notion of "hog rifle" just perfectly, but is likely has some other rationale/excuse behind it. I prefer the "hog rifle" notion).
Hog rifle: (as I interpret it) the family farm gun used to dispatch domestic hogs in hog-killing season and whatever other farm "business" and that also doubled as a hunting gun in a time when there was very little big game left around here. And folks didn't have a rack full of guns to choose from. Big game here is thicker now that ever because of modern agriculture (corn vs. acorns) and conservation efforts. They were of modest caliber. 45 would have been a "big" hog gun.
Also there are dozens upon dozens of old threads discussing calibers if one cares to look. In those threads one can learn from many great shooters who no longer make smoke and others who also passed this way before.
