I have some 3" cellular-core pipe in my garage, and I'm not sure what it's supposed to be used for. The side says it's schedule 40, be the pressure rating says **Non-pressure** (with the stars). I used it on my first and second APH, and it seemed to work fine, except my end caps on the PCs are now bulging out a little (I plan on retiring it). It's pretty thick, almost 3/8". Would it be ok to used this under pressure?

I wouldn't risk it for anything with pressure higher than a small homemade. It is non-pressure rated for a reason, and I would not take chances.

maybe use it to make homemade fragmentation devices?

I would definently NOT reccomend the use of cell-core PVC. "Cellular Core" , means that the actual PVC has bubbles in it. You've probably noticed that it's a bit lighter than usual PVC. Manufacturers do this to save material, and make the PVC lighter. It's perfectly okay to use in ground drainage situations or to transfer non-pressurized fluids, but it's a no-no in any type of homemade.

Cellular core pipe has a bunch of little air bubbles in the pipe, making it lighter. It also causes the pipe to be extremely weak for applications involving pressure. Don't use it.