There have been at least 2 companies that I can think of off the top of my head that make a 410/.45Colt Derringer, both in single and double barrels. There's a third company making a single-shot 410/.45Colt hunting handgun (Super Comanche, ims). One of those Derringer companies also makes a 5 shot "Pepperbox" 410/45 Derringer.
The leg, particularly the thigh & areas above the knee have lots and lots of really important plumbing structures. Injuries there tend to be very dangerous.
Big debate on this issue. Rule of thumb: Always Aim.
Generally, when you're chucking lead out of a shotgun that is heavy enough to penetrate to vital areas in a human, the lead chunks tend to be larger and fewer. 1 to 5, depending. When you have that few slugs/pellets you have a lot more chance of missing if you don't aim, and spread tends to be minimal, even when from a short barreled firearm. To get a good spread requires distance and a lot more pellets, such as some sort of Birdshot, which tends to not penetrate very deeply, creating shallow wounds which, while looking ugly, don't reach the vital organs which can cause a "stop" due to drop in blood pressure (nevermind CNS hits!).
Aim.
Peace favor your sword,
Kirk