hmmmmm...
I'd go for a small SMG, maybe a scorpion vz.61. It's a .32 ACP, blowback operated, I've never even seen a real one with my own two eyes, but I'm thinking it'd have very little recoil. And it can be kept in a holster like a pistol. It's sometimes refered to as a machine pistol. Not sure of the legal status of a scorpion though, and .32 ACP may be a bit scarce, but check it out anyways...
I heard that .38 Special revolvers are popular for self defence. If you're gonna get one, get it in .357 Magnum so you can use both .357 Magnum and .38 Special bullets in it.
BE CAREFUL when mixing .357 Mag and .38 Special - you CANT put a .357 Mag cartridge into a .38 Special gun, .38 Special can't handle the pressure, but the vice versa is safe.
That's what I'd go for, a scorpion and a 9mm revolver (.357/.38). But you wanted a long gun, so you're refering to shotguns or rifles yeah?
Any shotgun would be suitable for me, an over-&-under break action 12 gauge. Maybe a pump-action would be better, as it can hold more shells.
Shotguns are excellent, as you can put anything into a shotgun shell, with the right tools. Rock Salt is a nice non-lethal buckshot-type projectile that won't penetrate, but'll make the victim sting and he'll be in pain for a while!
A 12 gauge is quite a bit of kick though. Maybe a 20 gauge or smaller would be better, less recoil. But you can get low pressure ammunition for minimal recoil, I think...
If we're talking rifles, any .223 caliber rifle will have little recoil as I believe, and will be lethal enough at close range, which is good enough for self defence. Maybe an AR-15 or similar gun would be a good choice.
I've heard of a gun called a Bushmaster Carbon-15. It's an AR-15 with a 10 inch barrel and rails for sights, quite small. For self defence I'd get me one of those. It has no buttstock, although the the receiver extention sticks out, so you have to use it like a pistol, or just shoot it without firing from the shoulder.
I'm thinking of practicality here. That means anything small enough to easily move inside of a house. shotguns are good enough for self defense.
I've only ever shot handguns, and the only gun I've mentioned above that I've actually shot is the revolver, a S&W model 586. Beyond that I'm just talking about what I know about guns in general. I'd feel very comfortable with a shotgun for defence at point blank range.
Those are just my thoughts...
Here in Australia, self defence is not a valid reason to get a firearm license. You're lucky that you can go to a gun store and get anything you want in there. God Bless America!