I would suggest a 20 ga if you are thinking about shooting sabots out of rifled barrel. Also have you looked at the single shot H&R/NEF guns. I would think they would work pretty well for you. Mine is pretty short compared to an 870 or 500.

I don't understand what you are asking about when you say key lock? Are you asking if they come with a lock? Most guns you buy now a days will either come with a trigger lock or a cable lock, which is basically a pad lock with a long cable that goes thru the action. They do not have a lock built into them if that is what you are asking. And in most states if they didn't come with one you would be required to purchase a trigger lock. Heck they give them away where I live sometimes.

I am with the others on the 870's as well, the price differences are just in the finnish, they are all built about the same inside. You also might consider a browning BPS, the have a bottom eject and to me a smoother action than the 870. They cost more too though. They also have a better twist rate for the rifled barrels which means they may not be as picky about ammo. Slug guns can be pretty finnicky about what they shoot well sometimes.

I honestly think your best bet would be a used semi auto with a slug barrel Like an 1100 or 11-87. Remington also has the new SPR 453 which will shoot a 2 3/4 inch shell up to 3 1/2 inch shells and is semi auto. It is imported from russia and retails around 300 bucks new. I have been hearing pretty good things about it. The only down side is if it were to break you would probably have a heck of a time getting parts for it. Where as an 1100 or 1187 would be much easier. With no hand to hold onto the foregrip recoil may be an issue for you, and an auto would give you the least amount of recoil.

Something else to consider would be a ported barrel. I had a mossberg with a ported barrel. I didn't notice some much a reduction in reciol, but it really helped with muzzle flip. They are pretty loud though, not as bad as a centerfire or handgun with a muzzle break, but they are louder than a normal shotgun.

Like I said though, your most inexpensive route would be a H&R single shot. Simple to use, very sturdy and fairly accurate. I love mine. And they cost around 200 bucks, some a bit more, some a bit less. Depends on what model you want. Depending on what hand you are missing I would think it would be easy to operate for you.

Good luck,
Paul