RE: .410 on deer?
Well I just looked up the ballistics and they are around 1800-1850 fps for a half ounce slug. That is like 219 grn I think. That is better than most hand guns unless you are talking about a hand cannon like .454 or something.
I would say that puts you well into the modern muzzle loading range for power. Imagine shooting a conical out of a .45 cal muzzle loader with a decent charge behind it.
I would say the limiting factor would be accuracy. If it shot well out to 60 or 70 yards I have no doubt it would kill a deer. Better bullets would help though.
***EDIT****
I miss read the text, it is a 1/5 (one fifth) ounce slug, not one half. Being around 85-90 grn, not 219. This is much weaker than I thought. I will have to do some more comparisons, but off the top of my head I would say closer to a .38 hand gun round.
Paul