I was curious about the same thing and found a link that has some info.
http://www.4-10.freeuk.com/mwdefence.html
It's a .410 page, so I would think they've done the research. The following excerpt makes it sound adequate, especially at close range.
SLUGS: First, let us look at the common .410 bore slug. The traditional .410 foster-style slug has a 1/5 ounce or 87.5 grain slug at a muzzle velocity (MV) of 1815 feet per second (fps) and developing a muzzle energy (ME) of 640 foot pounds (ft. lbs.). More recent offerings increase the weight of the slug to 1/4 ounce at a slightly reduced MV of 1755 fps for an ME of nearly 750 ft. lbs.
BIRD SHOT: The 2 ½ inch .410 carries ½ ounce of shot at an MV of 1200 fps. Fine shot sizes available in the short shell range from number 4 to number 9s. The three inch .410 shell carries 11/16 ounce of shot at 1135 fps and sizes range from 4s to 7 1/2s. Bird shot can be very effective, but its effectiveness, or rather the nature of its effectiveness will depend on range.
At contact range, either shot charge will act more like a frangible bullet than a pattern of shop and will deliver enormously destructive wound, The 2 ½ ounce shells deliver 700 ft. lbs. of energy and the three inch shell 860 ft. lbs. of energy. Such forces are comparable to a using highly frangible bullets in .41 and .44 magnums respectively.
While it's not a 12GA, I know it hits a lot harder that the 9mm I've got in my nightstand.
Of course, I've got 14 follow up shots. YMMV.
RT
