This is what I did: (note, this is for an inline blowback, not a VM68).
I know, not very pretty. I was trying to emulate the CooperT effect by focusing the blast to the base of the paintball.
It does not seem to work as well as the CooperT, and I think it is the SHAPE of the CooperT bolt face. The CooperT is curved to match the shape of the paintball. This way the blast is DIRECTLY focused onto the rear, causing the backspin.
My version above is a more traditional bolt face. So much of the blast probobly gets dispersed, and I'm sure does not create as much backspin.
Consider the bolt "rimfire" PGP. I'm sure this is where Chauncey got the idea. The original PGP had a primitive bolt, where the valve emptied directly into the firing chamber, bypassing the bolt. You would think this would create some sort of backspin, or forespin, but it doesnt. That blast much need very specific focusing.
nick