A good friend has a Remington 870 he keeps by his bed. It's loaded with birdshot in case evil steps through his door. In about a month, that shotgun was used to harvest a black bear and a wheelbarrow full of squirrels. The only thing my friend changed was the ammunition. If you know what lies ahead, it's simple to stuff the right ammo in your shotgun. For hunters this is easy, but tactical situations require flexibility—and flexibility is the shotgun's first name.
There's a wide variety of shotgun loads available for most any task: birdshot, buckshot, slugs, tear gas and flash-bang projectiles, even rubber bullets and beanbags. With a pump shotgun like the 870 or a semi-automatic like the 20-gauge 11-87 behind my office door, you can transition between loads within seconds as the need arises.
Let's say you are accosted by a couple of thugs intent on taking your money first and your life second. Both are armed. One is at almost spitting distance and the other—the lookout—is about 40 yards away. Your shotgun is topped off with buckshot, but you have slugs on the sidesaddle shell carrier.
Your response? Shoot the closest threat first with buckshot. If he is incapacitated, transition to the second bad guy, but while doing so, slip a slug in the magazine tube. Whack the distant aggressor with the buckshot and if he is still functional, hit him with the slug. This drill is similar to a failure drill with a handgun and could be applied against any threat at any range where buckshot failed to deliver the desired result. Bad guys can wear body armor, too, and smart ones get behind things when they are being shot at. The idea of switching to the slug is it has a tremendous advantage in terminal performance over buckshot, especially at longer ranges.
To practice this ammunition transition, top off your shotgun (pump or semi-automatic) with buckshot.
Put a few slugs in the sidesaddle ammo carrier or your pocket. Using a shot timer, engage a silhouette target at 7 yards. If it's a pump, cycle the action. Without removing the shotgun from your shoulder, shove a slug in the magazine tube with your support hand. While doing this, keep the shotgun oriented in the direction of the threat(s) with your trigger finger alongside the trigger—ready to shoot if the need arises. You may have to shoot the close target again, or you may need to shoot at the farther threat with buckshot before you get the slug in the gun.
For scoring purposes, the silhouette target at 7 yards should have a hole for each buckshot pellet from one or two rounds; depending on how many times you shot it. The 40-yard target should have a big hole in the center mass from the slug. If you elected to fire the second round of buckshot at the 40-yard target, it should have holes for at least a third of the pellets from that round. Get all your hits in less than 10 seconds and you're average. Complete the drill in less than 7 seconds and you're dancing the shotgun samba.
If you're going to use a shotgun for self-defense, take advantage of its versatility. Experiment with different chokes and loads to find the combinations that accurately deliver the munitions you have on hand. You won't have the luxury of swapping chokes or adjusting sights in the middle of a gunfight, but you can still be flexible by learning to quickly switch from buckshot to slugs or vice versa.