I would not use a .40 for deer, even if it was legal in the area, and it is and so is a .25 or .32. I think .45 is on the light side
as I simply think a .50 is better in all regards (hunting and trail walks) as well as being better for trail walks. The smaller the
calibre the more lost targets you will get due to those 'scoring' not hearing the hit. A .50, is a big step up from a .45, which is
better than a .40, which is better than a .36- etc, etc - for trail walks.
If I wanted or had the opportunity to hunt a lot of small game, I'd get a .36 (which I have for small game and the squirrel
shoot at rendezvous.) The .40 is the upper calibre allowed for the squirrel rifle shoot at rendezvous. They can be very accurate,
& have the potential to be more accurate than the .36 but the targets on the course are small and move well with the small calibre
ball hits - so small dia. lightweight balls are not a hindrance to scoring well on THAT course of fire.
The larger the ball, the better accuracy at the longer ranges - it's all relative.
A 64 pound ball from a cannon is more accurate at 2 miles than is a 12 pounder.
A .50 cal. rifle has the potential to be more accurate at 100yards, than anything smaller. As with any muzzleloaders, accuracy comes
from the load combination. Unlocking the potential is the shooter's job, as long as the gun is up to the situation. Choose wisely.
A local guy here shot a couple whitetails with my .45 round ball load. For a test, he switched to using the REAL bullet from a Lee mould
and shot a couple more deer. The end results were identical - dead deer and identical performance from what he could see. The fact I
would not use a .45, but prefer a .50 (or .69), is simply a matter of choice. I sold my .45, never having shot a deer with it, but I know it would
have worked. It's "accuracy" load with oil lube was putting the 132gr. ball out at over 2,200fps. Within 100yards, it was not only accurate,
but would have been deadly.