I assume you are asking about what is known today as the 22RF CB? The proof pressure would have depended on when the firearm was produced. The rimfire has been around for a long time. It has gone from black powder to semi-smokeless to the early nitro powders into today's smokeless powder. SAAMI does produce a document dedicated to the rimfire. http://www.saami.org/Publications.cfm

I can tell you that the American standard loading for the CB using black powder was a 30 grain conical ball, 2 grains of black powder with produced m.v. 650fps with a m.e. 28ft.lbs. The early American smokeless produced a mv 720fps with a m.e. 34ft.lbs.

The current SAAMI Maximum Average Pressure for the 22 LR, 22 Mag, 17 Mach 2 are loaded to 24,000 psi Maximum Average Pressure with the Piezo system. This is according to recent email from CCI.

Pete