I had some time to pay yesterday with my 37mm Spike's Havoc and was looking for stuff to launch.  I had a couple 1/2 white plastic pipe end caps and 1/2 clean out plugs.  They screw together and if you dremel off the ribs for a wrench to tighten the cap it fits perfectly in my 4" casings.  Not a lot of volume for smoke or anything, but launch nicely and look cool loaded with the round top.  I think I'll dremel off the writing on the next ones I put together.  Also noticed if you tighten them hard with 2 of the black rubber o-ring gaskets that come with the cleanout, they (the o-ring) budge out and make it so you don't need a rubber band for a seal.

That's interesting.

I always wondered what people used for training ammo for those....

Honestly it's more of what can I launch for fun today, than training..  Some things fly decent and others barely leave the yard...  These tumbled but are not that much taller than they are wide so it went further than I thought.  I would love to mold something with fins like the specialty ammo projectiles or the spirals like some of the reloadableshells.com stuff without spending $14 each.

Thought about putting some resin inside the cap? Let the resin dry and it will add weight forward to give it some stability and it will allow it to fly farther...

Just a thought and I dont know what the resin will do when the round goes off.

I thought the same thing..  The pipe endcap seemed heavier than the plug back end so I thought it would fly straight.. Maybe it just needs a little more weight, or it's like a rear drive car and with the force in the back it makes the rear end pass the  front????  There is a lot to test and learn with these, but something fun to pass time and not a ton of money after the $675 for the Havoc, stock setup, and a couple rounds to play with.

Just be careful of what you launch from your unredgestered launcher. If it can be used as an anti personal round including things like baton or beanbag rounds you can go to jail for long time. Legealy you can only use things that are used for signaling some way.