While a .32 will kill deer with a proper head shot, I wouldn't try it...I have killed several with a scoped .22, but I started squirrel hunting in the 60s and have killed hundreds with a head shot...The difference is that a scope makes shot placement much easier and most of the deer I shot were when I was much younger and they would come through eating acorns when I was sitting there squirrel hunting...This is not something that most hunters should do...I grew up in the country and we killed 20 pigs a year, dad would dispatch them with a .22 pistol, hit in the head, they would drop...
Now when it comes to black powder, I started in the mid 70s with a custom made .45 that I used about 15 years...I used a 75gr charge of FFF Goex and killed 25 or so deer with this rifle...A .45 ball will go through the lungs and stop on the off side, under the hide...Deer usually fall within 75-100 yards when hit this way...The problem isn't in the killing, it's in the tracking...With no exit, this is more difficult...Remember...The early settlers were hunting for food, so if a deer walked up, they would take the shot...We don't have much feedback on how many they lost and I can guarantee you that they were better trackers after the shot than we are...So it is totally different...
Yes, you can kill a deer with a .32-.40 caliber muzzleloader, but after losing a few deer with the .45, I built a .54...Now, I am getting an exit hole to assist me in tracking...The .45 has been converted to a .40 and it is used for squirrels, with 25grs of Goex, it is a tack driver and does the job with head shots on squirrels...