RevolverRob weighs in with his take on defensive shotguns. Good read.
I’d like to add to the pump vs. semi-auto debate.
Pumps are simpler machines, run almost any ammo you can feed the gun, cheaper to purchase, can handle some abuse and neglect better. But they will also recoil harder and are a more complex manual of arms to operate. Semi-autos have a simple “point and click” interface, and you should be taking the time anyway to find the best ammo for the gun (that works, that patterns, etc.) so the ammo finickiness shouldn’t be too big an issue.
Think about the operator of the firearm. If it’s me, I could care less. Pump is fine. I’m a big guy, the recoil won’t bother me, operating the action is just fine and dandy. Now let’s take a small statured woman or even a child; something with less recoil could be beneficial to them (consider low-recoil rounds as well, which are more than adequate for home-defense shotgunning). Is the operator someone who practices? If so, pump might be OK as there’d be less risk of short-stroking. If not, the simpler interface of “point and click” from a semi-auto may serve them better.
There’s no blanket answer here. Merely adding more things to the mix for one to consider.
Speaking of all of this, KR Training will be having another Defensive Long Gun course on January 23, 2010. I took it last year with my AR. I wonder if I might want to take it again but with a shotgun. Hrm.