Your gun, your rights, your problem? It's pretty common in America to let someone borrow, use, try, or otherwise handle a firearm. Hunters do it in the woods, shooters at the range, purchasers at trade shows, and kids at summer camps.
Put those scenarios to one side, then consider the other side: criminal defendants arguing about who used whose gun to shoot so-and-so, or an otherwise responsible owner having to explain how his gun ended up in a kid's backpack at school.
So what's the law on letting someone borrow your gun?
America's Patchwork Gun Laws
There's an old legal adage that everything is legal unless prohibited. While it's not necessarily true, it's a fairly good guide when it comes to gun laws. According to the U.S. Supreme Court, the U.S. Constitution grants individuals a right to possess a firearm for lawful purposes, and this applies to states as well.
Federal Gun Laws
Federal law bans anyone convicted of a felony from possessing a firearm. That's one of the more common federal criminal prosecutions out there. It's also illegal to ship a firearm out of state without a license. Certain types of firearms - assault weapons, military grade hardware, etc. -- are either banned or tightly regulated. It's important to know who you'd be giving your gun to. Note any specific laws about the type of weapon as well.
State Gun Laws
From there, it really depends where you live. State gun control laws vary considerably. Buying, selling, or transferring ownership of a gun might be regulated where you live. Virtually all states prohibit possessing a gun near a school. Big cities and urban areas may have more restrictive policies than the countryside. Gun laws are for the most part state and laws, and it's difficult to generalize.