garywms,
shooting any shell in a gauge reducing adapter will not increase pressure. If anything, it likely reduces the peak pressure or doesn't change peak pressure. But the relavent thing is the stresses that the gun barrel sees. A 410 shell, producing 11000 psi, fired in a 12/410 chamber adapter, will not exert anywhere near the stresses that a 12g/11000 psi shell will. As the diameter/gauge increases of the shell being fired in a chamber adapter, the difference in stress from a shell fired in the adapter will be closer to a full sized shell of the same pressure.
