I probably shouldn't comment on this thread since I've never shot a deer with a .40 and don't presently even own one (hope to rectify that soon). That said, I suppose my opinion is as valuable as the next guy and all I've seen on these threads are opinions anyway, so here goes.
Most of the folks espousing the use of .40s for deer on these threads seem to make mention that it's all about bullet placement, which I'd obviously agree on. Many times they go on to point out that they shoot all the time, practice constantly, shoot X number of shots per year, are expert marksmen, etc.. Well, I wouldn't call myself an expert marksman with a flintlock but I'm better than average and I've killed my fair share of deer (a couple dozen) with .50s and .54s.
What I find odd is that I've shot many deer right through the boilermaker with my .50s and .54s (I typically use about 70-75 grains of fffg with .50 or 85-90 grains of fffg with .54) and have had lots of deer take off carrying the mail after being hit. Sometimes they bleed well but they oftentimes don't. Here in PA, our late flintlock season, when I've killed many/most of my deer, usually has snow on the ground and I've not lost many deer over the years but I rarely get drop dead in their track kills and oftentimes need the snow to help track and find my kills.
I'm also willing to admit that I have made a few less than perfect shots at deer, for whatever reason. Hey, I'm human and there are lots of environmental factors at play during deer season. I'm glad to say that most of the time, I've been able to recover my game because the larger calibers did a better job on the deer even when I failed.
Those are just my anecdotal observations and anyone can draw whatever conclusion that they want from my post.