Actually, you do need good markmanship skills to hit the target (in this cas a deer) with a shotgun.
For example it is very challenging to hit a target at 50 yards using a 12 gauge shotgun with rifled slug. If using a sabot slug with a fully rifled barrel the marksman (depending on his or her skill level) could have good accuracy at 100yards.
Regarding buck shot, A 12 Ga with 2 3/4 inch shell and 00 buckshot contains typically 12 to 16 individual shot. Thus, you really do have to aim in order to hit your target. It is a good idea to think of your shotgun as a rifle, so aim.
For prcatice on a moving target I suggest finding a public range with sporting clays. Try for yourself to see how easy or how challenging using a shotgun is.
You can find out more about safe gun handling and good marksmanship at the NRA web site.
nra.org/
Also look up you local shooting range in the yellow pages.