I had an interesting experience with an AK-47 the first time I fired one. I had cut some 4x4 blocks from a cedar post to use as targets, put them on the ground about 75' away, and emptied a full magazine one shot at a time. It didn't seem as if I had hit anything until I looked at the blocks and saw that they were riddled with holes. Apparently the slugs went so cleanly through the blocks they hardly moved them at all.
Apparently the slugs went so cleanly through the blocks they hardly moved them at all. I use something frangible- I have a box of clay pigeons just for the purpose.
Sealed cans of your favorite carbonated beverage are fun, too. :-)
Tethered helium balloons will move with the wind and make a more challenging target. Actually, this might be grist for a good vanity one of these days- what's your favorite improvised plinking target?