Well you see some folks are pretty new at this type of firearm (black powder round ball), and they try to equate the modern stuff that they know to what they don't know about the antique stuff. There is some confusion of projectile weight being the only factor, when it's weight, velocity, bullet cross section, and bullet composition that are the determining factors at impact, with accuracy being the determining factor prior to impact.
It [.40 round ball] works fine on Deer if you get close as in archery hunting
One of the problems with something like this quote is many folks don't understand what kind of range(s) we are talking about. 40 yards is the max for archery (imho) and the archers that I know who shoot at those ranges miss a lot of deer, and track a lot of deer. I'd say proper archery range is at 25 yards or less, while a .40 will reach out to 50 yards and kill deer with a 60 grain charge of 3Fg, with a standard heart/lung shot and a pure lead ball. On another forum, folks were upset at a popular writer who used a 50 grain BP charge with his .490 ball (iirc), but the critics paid no attention to the range that he chose to engage the deer, which always makes a difference.
(Note I mentioned the location of the impact with the .40, and the type of projectile. For example, go for a neck shot, or use a .390 ball made of wheel-weight alloy, or both, go for a long shot, and you change the parameters.)
Range vs. animal are always factors in ANY hunting, whether archery, or muzzleloader, or cartridge/shell firing gun. No matter what you use, a .45 caliber flinter or .458 Remington Magnum, you will reach a range where the bullet will not do the job. A hunter must know the limitations of the load that is being used in both accuracy and impact.
Many hunters, especially in the cartridge community [I have observed], try and substitute caliber and energy for accuracy and shot placement (imho). I have had conversations with folks who hunt Western PA, The MD Panhandle, and Northern WVA who say a .30-06 is mediocre, and a .338 magnum or 7mm Magnum is the way to go, while scoffing at the .270, .25-06, and the .30-30. They have told me that my .54 round ball just won't "knock 'em down", though none of them ever tried to hunt with one.
What is interesting I find is that some in our own group swear by a 125 grain .440 ball, but disdain a 90 grain .390 ball, which is a difference in weight of a .22 short's projectile, and a diameter of difference of a mere .05, with no discussion of powder charge, bullet material, shot placement.
The original question was about the advantage of using the same load for squirrels and for deer with the .40, and that being an advantage as one would know from long use where the bullet would go when aimed, and so not have to practice with two different loads. I think the popularity in one region of America of the .40 was due to it's accuracy allowing the user to be very confident of shot placement, while being very frugal in powder and lead use. So YES, it would work, if you knew how close you need to get the deer to allow for a quick, humane kill. In Maryland, today, the question would be moot, as the minimum powder charge is 60 grains of BP for deer, which is
screaming fast in a .40, and a little hard on a bushy tail.
LD