The so-called Pressure rings are the modern equivilent of the PAPER PATCH! The rings are cut into the bullet down to the BORE DIAMETER, and the tops of the rings are slightly higher than grouve diameter. This allows the metel in the rings to be easily displaced into the cut in the bullet, acting similer to a soft bullet!

All bullets expand the barrel as it passes through the bore, sort of like an egg passing through a snake. The bulge follows the bullet, and retracts as the bullet passes. This is the problem with Nono-metel bullets in doubles. As Micky says in a single barrel, the elasticity of the barrel lets the barrel react to this without damage, but in a double rifle the solder between the barrel and ribs sometimes lets go! The Valmet, and Tikka do not have ribs, so are not adversly effected. The pressure rings ar the way to go with the X-bullets, and Monolithic bullets if they are to ever be used in doubles, without risk!