I have a Remington 870, pump action, shortest legal barrel. It holds 4 shells in the tube (really, if you're at home, and you need more than 4 shells -- you're fucked anyway. For me, at that point, my wife has grabbed the AR-15 as well...)
Here's how I have it loaded:
Hornady Zombie Max 00 Buckshot
Hornady Zombie Max 00 Buckshot
Hornady Zombie Max 00 Buckshot
Remington 1 oz Slug
Hopefully none of those shells get fired. The slug in particular is really bad news for an assailant -- 9.1 inch temporary cavity.
1 oz slug in action against ballistics gel
Shot shells in ballistics gel
That second video is why I wouldn't trust birdshot to save my life. Look at the penetration difference -- even the mighty (for a handgun)
.45 is no match for 00 Buckshot.
Second video recommends #4 shot actually, I might have to think about reloading my shotgun. I have a couple of boxes of magnum brass pellet #4 shot... maybe go #4, 00, 00, slug.