On the latest Pinky and the Brain and the Three Stooges:
1. Ping-pong ball slows down as it approaches the closed end of the (mostly-) evacuated barrel. Duh. Even without blow-by, there's some air ahead of the ball, which will be compressed. The packing tape holds it in. I'd assumed that the evacuated-barrel ping-pong-ball accelerator would have the end closed by some little disc held in place by vacuum (OK, by outside atmospheric pressure), so oncoming pressure buildup would blow it off.
2. I'd also kinda assumed that the logical step after the pressure gun was just to evacuate the barrel of the same old pressure gun. Did they try that and not show it, or...?
3. In the final encounter of supersonic ping-pong-ball vs. porcine appendage, the high-speed footage made it look remarkably like the air stream was doing most of the damage. This would be consistent with my suspicion at the beginning: yeah, a supersonic ping-pong ball would be dangerous, but whatever's keeping it supersonic is probably more so. And, if you're in an environment where a ping-pong ball can continue moving at 1100 MPH for a nontrivial distance, you've got bigger worries than ping-pong balls.
4. If reinforcing materials were allowed for the cannonballs, why not for the tube? Why not make the cannon out of Pykrete?