The larger the rim, the greater the pounds per square inch against the bolt face, with a normal tapered round. The .30/30 has a rim size of .506" while the .303 has a .540" rim. As the Brit has much greater capacity, it will therefore develope much better ballistics.

Many of the newer fat ctgs. are quite straight sided with sharper shoulders and are loaded to what is now considered normal max pressure of 65,000PSI. The older standard tapered magnums like the 8mm Rem mag. and say, 7mm Rem, .300 Winchester mag, with a .532" rim have similar SAAMI maximums as the .338/06 and .270 Winchester which have even more taper and a much smaller .469" rim.

Suffice to say, the magnums have less taper and therefore the chamber area takes up more of the pressure, ie: less bolt thrust due to the case sticking in the chamber better- than the more tapered round. The '06 sized rounds have smaller rims which offsets their greater taper. Ballistics are for the most part, a juggling act of characteristics.

The more taper on the case walls, the greater the bolt thrust.

The larger the rim, the greater the potential for more bolt thrust in pounds per square inch.

How much bolt thrust is increased depends entirely on the case taper and chamber condition, just as it does with any other case.

A very straight sided case with a huge rim, can have literally no bolt thrust at 55,000psi, like a .577 IMP, or if heavily or excessively tapered like a NE case, produce excessive bolt or breech thrust at only 45,000psi.

A very small diameter rim of an improved type case, say .218AckBee can have a maximum of, say, 62,000psi due to it's .408" rim adding immense strength to what is a very small diameter case, but taper that round excessively like a factory .218Bee and it might produce excessive bolt thrust at 50,000psi - higher than the bigger NE case, due to the PSI against the breech being less due to it's smaller diameter.