If you want to burn cheap ammo in a hurry, then 10/22 is the way to go. For something with a little more punch, then pistol cal lever action is hard to beat. I have a M94 in .45colt that is a blast to shoot, and I've never had jamming problems with it. With reloads using cast bullets it doesn't cost much to shoot, and hits the 100yrd silloutte with a solid thwack.

Semi's are fun too, Mini-14 or SKS, little recoil, fairly cheap to shoot, and a bunch of fun.