I did it once, with a good friend, I got it back about 20 weeks later, and the barrel was covered with rust. He got it wet, and put it back in the plastic/foam carry case with the water on it. The barrel from a visual standpoint is now junk. The rifle was in "like new" condition.
Fortunately it was only a 10/22 and not a real gun (read expensive). Probably the same reason I would never loan out any of my real guns.