Smith and Wesson 500
You will haver to define if "biggest" means caliber or overall size.
It depends on whether you mean legally, or what is the biggest handgun available, and whether you mean handgun size, or caliber size. The largest commonly available caliber is .45 ACP, however, .50 AE is also fairly common. That said though, there is also a handgun available in .600 NE. Most states don't have restrictions on caliber, however, some may.
You will have to be more specific. Handgun, shotgun, rifle, crew served?
The biggest reason is that it is illegal (in most of the U.S. anyway)
The smith & Wesson .500 or the Pfeifer Zeliska .600 Nitro Express revolver its a debatable topic
It depends on where you live. In the U.S. in most states, there is no specified limit. In Tennessee where I live, "handgun" is basically defined as any gun designed to be fired with one hand or something very close to that.
Books have been written on this topic. Find a copy of Cartridges of the World for the most common handgun, shotgun and rifle.
If you have a handgun carry permit, definitely. The handgun carry permit covers "any handgun that you legally own or posses."
why can you get a shootgun at 18 but not a handgun
S&W .500 magnum. how about the 45/70 revolver ??? or the 600 Nitro Express
A regular handgun, no. There is no registration in TN.