Some form of a gas stripper, just larger than the bullet at the muzzle, to pull the gasses off it's base, and redirect them backwards down the shroud over the barrel.

With air rifles, there are no baffles inside the shroud around the barrel, but a series of baffles ahead of the muzzle before the end of the shroud/tube.(4 to 6" more than actual barrel length)

The shroud needs a couple holes around it's periphery at or near the action face, to relieve pressure.

This again, if for air rifles, but a similar system would work on powder burners, if baffles were also placed in the shroud, I would guess.