Toadflax
It is indeed legal to shoot somebody elses rifle/shotgun provided that they are with you and they have the right to shoot on the land you are on as to the legality of shooting animals in the USA, you would be Very surprised at how strickly monitered they are on this,as an example in [most] states you are only allowed to take 1 species (antlered) of deer per season and the seasons are VERY short, then they have differant class of "weapon" eg theres Bow season,muzzle loader season rifle season and so forth.
The impression i get from non shooters over here of the yanks is that they grab there "gun" and head to the hills for dinner, this actually couldnt really be further from the truth,I usually visit the USA twice a year and Canada once and in speaking with the others in camp they are astounded at how long and leniant our game seasons are.
Dave