[Double Naught Spy]

I am thinking that bird is too light since at 15 feet you are not likely going to have time to get off a second shot if the first doesn't stop your nocturnal visitor and he comes at you. if your first shot doesn't at least stop him for a quick count.

I don't think birdshot is a great idea either, but at 15 feet you should still have time to get off at least one more shot. Assuming he charges after the shot, then you would have about a second to deliver the next shots. If you are proficient with a pump gun, you can get off two more aimed shots in that time (yes, I know some of the professionals can do more, but we are talking about regular folks and not professionals, I am assuming).

With a semi-auto, you should be able to get off three shots in the second followng your first shot.

If the guy is already running at you and you shoot with he is 15 feet away, with a pump shotgun, you should still be able to get of a second shot before he reaches you and at least 2 with a semi-auto.

I don't want slugs or heavy buck hitting the guy next door.

I have a feeling that the structure of your home and that of your neighbor's home as well as distance away form yours are going to be a very relevant issue in determining what you think is safe enough to not harm your neighbor. I am assuming you won't be shooting your intruder where he is standing in front of a window through which you can see your neighbor's window.

I have thought about and rejected less lethal because I want the first hit to end the threat.

Of the various pellet sizes for shotguns, common/typical bird shot (6 or 8 size) is probably the least likely to get the desired result you seek. Bird shot in that size range only gets about 5" of penetration (http://www.google.com/imgres?imgurl=...:0&tx=84&ty=67).

Size 4 bird shot gives about 7" (http://www.shadonet.com/?p=14).

markj suggested a compromise between bird and buck by going with size 2 bird shot. It will get you about 10" (http://www.google.com/imgres?imgurl=...w=1468&bih=670).

BB shot (0.18" diameter) will get you out to about 11-13" and so fit fairly well with FBI 12" penetration criteria. So if you were wanting to put the guy down with your first shot, then you are a lot better off with using a shot size that will give you sufficient penetration to get the job done.

From there, the next step up might be #4 buckshot(0.24" diameter). It runs about 12-15" of penetration.

So you don't want to shoot your neighbor because of buckshot or slug overpenetratin. Do you envision having the same concern with .22 lr ammo? If not, the you might consider using BB or #4 buckshot as the pellets individually will be performing in the approximate same realm as .22 lr.

YMMV.