Take the barrels, slides, and firing pins from several pistols, and reassemble randomly. The gunpowder may be tagged as well as the bullets and cases being marked, but shuffling your ammo with all your friends will mess that up too. Pull all the bullets from the cases and reload them randomly. There will be tags from random loads of powder, random bullets, and random brass assuming it's left at the scene, and the bullets and brass won't match. The brass will have breech marks from one gun, firing pin impressions from another, and the bullets will have rifling from yet another. Sounds like a big ol' mess for the CSIs.