4-10 is light gauge shotgun.
10 gauge is a much bigger load.
I have seen slugs bounce off of targets without doing any damage.
I would suggest that you stay with .12 ga as the ammo is commonly available and if you run out of ammo you will have an easier time finding it.
In most states purchase of ANY gun requires a background check if purchased through a dealer operating under a federal firearms license (FFL).
It is legal to purchase from any individual (Non-FFL) without going through a background check or licensing procedure. Unless you are a felon.
Personal Opinion:
The entire concept of gun registration is the most absurdly stupid idea I have ever seen. I have been to gun shows where licensed dealers have display tables next to non licensed enthusiasts. The dealers are required to do the background checks, etc. while the guy right next to him does not. Please tell me this is not a government run operation.
