What about those wee little deer in South Texas or the antelopes, wouldn't the 5.7x28 be a decent round for them?
Shot placement would be critical and the range less than 100 yards IMO. (Preferably 50) Miss a vital organ and all you'll do is create a wounded suffering deer that you can't find. That's not good at all.
That is true with other cartridges as well of course, but if "I" wanted to drop a deer I'd want more mass hitting the target, but this is just my opinion and I am in no way trying to insult the 5.7 round or anyone who would use it for hunting. You just HAVE to know where that round is going to hit and it needs to strike with enough force to kill or critically disable the prey with one shot. That's why praticing at different ranges is so important with hunting weapons. I just don't have the confidence in a 30 to 40 grain projectile delivery a "hammer blow" to a deer at any long ranges.
I would however use it confidently on smaller animals if the need arose. IIRC a PS90 sighted at 50yds will strike a target about 2.5+ inches low at 25yds and 4.5+ inches high at 100yrds. So I would have to be pretty sure of the range before pulling the trigger on a critter.
Now self defense is another story all together. Whether it is man or animal that is the threat, I probably would not be engaging the threat at 50 yards, I'll still be trying to resolve the situation in other ways (like evasion) unless I'm being fired upon or chased.
If I get into a situation where the threat is closer and action is required, I would be aiming at least 2 or 3 inches above center of mass and firing multiple rounds. I'll hit something vital.