[fso301]

I wonder what their gimmick really is? It can't possibly just be a brake.

Gas redirection of some sort. Any system that bleeds gas will reduce velocity.

[Bob J]

It doesn't have to bleed any gas, just redirect it to a termination at the the front or sides rather than the back of the cartridge (which then transfers the energy along the gun back to the shoulder/palm).

To do that it seems to me there would have to either be holes in the cartridge or in the barrel just ahead of the cartridge. These holes would dump the compression into forward oriented pockets. As long as the additional airspace was minimal, it might not affect the range significantly.

[Buffalo Head]

"Any system that bleeds gas will reduce velocity."

Only so much as velocity is reduced by lower gas pressure behind the bullet. The direction of vented gas is also important. Newton's laws of conservation of momentum still apply, i.e. m1v1=m2v2 where m=mass and v=velocity