An ordinary 260 modeling balloon is inserted into 4.5" long aluminum cylinder. Several solid nails are inserted through holes in the cylinder, into the balloon, and out the other side, without the balloon popping!
If the balloon is transparent enough, spectators can look inside the cylinder and see the nails in the balloon!
The balloon is ungimmicked, and both the nails and cylinder are completely examinable before, after, and even during the effect. Packs small, plays big!