I don't see anything wrong with shooting a mule deer at 500 yards. The big ones aren't dumb and if you are on public land, you'll probably have to take some long shots. I have passed on long shots that in retrospect I should have taken. Better to shoot a mature one at 500 than a 3-year-old at 100. And for some species in some places, you may not draw that tag again for a very long time, if ever. We could limit ourselves to muzzle loaders or archery but we don't.