Yes, earlier ratings for the old cartridges might not be the same as with the current CIP ratings. I suspect the CIP rating for the .303 is based on the later military pressures. I remember trying to estimate the pressures via the QuickLoad simulator and finding the .303 was at lower pressure than the .375 Flanged. Also, I see I used a base diameter too small for the Flanged. Based on just the CIP numbers, I should have reported 13% more thrust.

You mentioned that one should use the I.D. of the case instead of the O.D. While this should give a better number than O.D., I use O.D. to estimate thrust because it gives a more conservative number. For your calculations, based on tsi, you need to use O.D.

There's a good article in the April, 1967 American Rifleman by the technical liason for the old Kynoch. He clearly states that when reading pressures with base crushers (as was done with the tsi ratings you give), one must use the O.D. to compute the thrust. Indeed, this is how the tsi rating for case diameters other than that of the .303 was computed by them. Base crushers require the use of a well oiled case, and I believe the chamber pressure is being carried throughout the oil film, thus the need to use the O.D. Sadly, the fellow didn't state why one must use O.D., but I believe my guess is correct.

While not relevent to this discussion, he also noted that base crushers underestimate peak pressure more than the radial crushers (I would have guessed it to be the other way around), and of course both were found to be low when the piezo sensors were used. He also noted that dry chambers give thrusts about 3/4 that of oiled chambers.

Perhaps our friend should consider the fine old 400/350 instead. I believe I'm safe in saying it's thrust is no greater than the old .303 loadings.

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Uh, kindly disregard my blather on O.D. and base crushers. That obviously deals only with making the pressure measurement in the lab. One's gun uses a more or less dry chamber, and with that, I.D. gives a closer estimate of the effective piston area. Thrust is also affected by the yield strength and thickness of the brass a bit above the web. Comparing the relative piston areas using O.D.s shouldn't be too far off, though.