Well, I ain't by no means an expert on anything, but to me in a small bore a .40 cal is the .243 of Bp rifles. You can dog it down & it is accurate as anything you could want & bump it up & make it reall zip at 2000 fps & it is still accurate as can be. Of course shot placement is critical on any small bore when hunting deer, but the .40 is adequate. Problem is most guys are just plain lousy shots & they need all the help they can get, so a large bore is best for most..... But in any case, as 40 cal will do it IF you do your part.
As for owning them I have owned ? 4-5 .40 cals. that I can think of right off hand & all shot under 1" groups at 100 yards off the bench. As for a comparison I have owned 32, 36, 38, 40, 45, 50, 54, 58, 62 and have a 72 cal I hve not built yet. And I have owned several of each (except the .72) not just one or so, thus having more than the usual occasional shot with any particular caliber. My favorites for the small bore is the .40 & the favorite in the large bore is the .54 cal. The .54 in a ML is the 30-06 of rifles as a comparison. This is all just personal opinion but it is based on having owned ? 75 or so BP rifles in the past, so not an opinionated view from owning one or two calibers & hearsay on others...
Hope this helps....