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.
Stuff From Hsoi 
Thanks for the linkage, Hsoi. I didn’t really go in depth on the Pump v. Semi-Auto debate, because so much is out there. Your points here are excellent. In regards to pump v. semi-auto recoil, I’ve found that most people can handle the recoil of any of the guns in either common gauge, as long as the weight of the gun is somewhere north of 6-7 pounds.
Low recoil rounds can sometimes never be made to function fully in Inertia Driven actions and those guns already recoil as hard as any pump gun, which is why they are the first semi-autos I cross off the list for defensive purposes. Gas guns certainly recoil less and can often be adjusted to run lighter loads or low recoil rounds reliably. Now, here’s my real beef with all of this adjusting of the gas system to get loads to function properly. What happens when you want to practice with birdshot? Now you’re adjusting the action to function with bird loads for trigger time, and then adjusting back to get your gun running with buckshot again. If you forget to get it working and check it at the range, you could be running an expensive single shot gun if you have to employ it for a defensive situation. Malfunction clearing with most semi-autos is never a straight forward exercise either, I find the gun comes from the shoulder the bolt gets racked and now we play the “will the empty hull come out or get stuck” game. It doesn’t matter if you practice malfunction clearing, fired shell casings are longer than unfired ones, so there is the risk it gets stuck. All of these issues just drive me away from semi-auto, the less complex manual of arms for the shooter is a benefit, but I’m not sure it outweighs the increased complexity and known issues of semi-auto.
As for pump, it can suffer the same issue with a jammed cartridge as above, but with the pump, you can rack the bolt and shake the gun with it still at the shoulder. I’ve seen pumps jam in this way and I’ve practiced it myself. A good shooter will cant the gun (just like we use gravity to clear a stovepipe) and work the action with some force, which has increased leverage thanks to the pump. Most of these stuck shells come unstuck with just this method. You get the increased leverage here and I think it’s worth noting.
It’s also worth noting that like you mentioned pumps tend to work with ALL loads, ALL the time. That’s worth it to me, because what if the fight doesn’t go your way and you’re looking for ammo and can’t find the buckshot you need but find birdshot? I want to put it in the gun and go, not call time out to adjust a gas system.
Just a few more cents on this debate. Ultimately, I think many people are better served by a pump and some practice. The semi-auto doesn’t aim and shoot itself, so it still needs to be practiced with as well.
-Rob
Certainly in agreement with you. I myself feel the same way, and why I tend to prefer pump. Now if I was a bird hunter, I’d probably want a semi-auto, but that’s a different application.
Still, in the end it’s just best to know all the pros and cons, the realities of things, and pick what’s going to work best for you. Then practice and train. Then train some more.