You gentlemen have painted a picture the to be quite frank, has left me sick to my stomach thinking about the possibility of a lingering death from using the .40 as a deer rifle. I will have to stop using it for deer. I love to hunt them and see them (as long as it's not on the road) we have a population problem hear, way too many.
So how are .58s?
Rich, for what it's worth, I've got a little different opinion about what is or isn't appropriate for game afield. You seem to have taken the thread as contrary to recommending a .40 for deer, but I'm not reading it the same way. I don't argue that larger calibers are not suitable by any means, but experience has shown me that reliance upon numbers found on paper are a poor litmus in determination of suitability for what one uses to hunt. No comment rendered here on the legality as prescribed by different states either.
As previously mentioned, muzzle loaders offer flexibility for hunting. They may be loaded down or up as the shooter deems appropriate. To the point of what is used, inclusive of caliber and load, one must first be able to put the ball where it will result in a decisive kill. In context of the original post that started this discussion, there are two eventualities here. One is that you smack a squirrel with far more oomph than necessary and in such manner there is little left but red mist. The other limits the shooter to precise placement and doing little if any damage to the edible portions by way of a head shot. Apologies to those who eat tongue and brains. In any case, if a head shot is successfully made, it matters not what caliber or charge is used, only that it be accurate enough to enable such placement.
I find little difference between head shooting squirrels and taking the necessary steps to down larger game with a round ball. First requirement is placement and that requires one to know the gun, load it appropriately and use it within one's limitations. Secondly, there is a requirement that the ball perform within certain parameters, ie. be reasonably predictable in the terminal phase. That means that it tracks more or less in a straight path thru the animal and neither over or under expands. Barring deflection by heavy bone, round balls do a fair job of the former if they don't deform erratically. Brittle alloys or very high velocities can contribute to such errant travels. Lastly, the ball needs enough momentum to reach the vital organs and hopefully exit on the off side if the shooter's intent is to ventilate the cardiopulmonary system completely.
In very brief summary of round ball ballistics...they suck....as compared to conicals. Round balls do not begin their journey with a great deal of energy, and will shed that quickly. Regardless of that they are quite lethal and they tend to penetrate far better than most of the unwashed masses would imagine.
There was discussion earlier in this thread that compared the .357 mag to a .40 cal round ball. In my opinion, it is an apples and oranges comparison on the face of it, but if one looks into the issue a little deeper, maybe not. Sectional Density is a measure of mass to cross sectional area and is used in the calculus of exterior ballistics and terminal ballistics, in the latter case as a comparative measure of a bullet's ability to penetrate. A 158 grain .357 slug has a sectional density of .177, and a .40 caliber round ball is about .102. For comparison, a .22 short bullet is about .105. None of these numbers are impressive on the face of it, but where it gets interesting is what happens when they strike a game animal.
Bullets usually expand or in some cases fragment when they penetrate flesh and bone, and when this occurs it very rapidly degrades sectional density. Bullets expand in relation to their hardness and impact velocity: Hard = less expansion, while high velocity tends to increase it. It is a world of nearly infinite variables and difficult to pin down precisely, thus we look for something that is reasonably predictable.
In the end, it is up to the shooter to decide what will or won't work for any given circumstance. A previous poster said the .357 was good for deer. In the hands of a competent shooter, I agree. So is buckshot when used within its limitations, and the shot size is smaller than .40 caliber in all buckshot. I've seen #3 buck (~.25 caliber) pass thru and thru on mature hogs more often than not. I've killed over 60 hogs with CB shorts, with one exception, all one shot kills. So, is the .40 caliber round ball adequate for deer? Yes, if you can put it where it needs to go. That means knowing your rifle, your quarry and yourself. If you aren't up to speed in any of those departments, get a larger caliber. Or, you can learn what you need to know to address perceived shortcomings. One thing is certain however, one can wound and lose a deer with very large caliber muzzle loaders, just like the small bores. Happens every year this time because a lot of folks just think a larger bore relieves them of the need for holding up their end of the bargain. Ain't so.