Good advice. By all means do measure different types of brass. In fact it might be a good idea to do so before writing about it.
I have measured a bunch of different cases and virtually all have a web thickness of .200 -.210". A notable exception is some of the later 308 Norma and 7x61 Sharpe&Hart. these are down around .170. Everything else, from 45/70, 30/40 Krag, 303 British and the "very modern" 300WSM, measure from .195 -.210. The variation as much related to the brand as anything else.
Brass hardness, as mentioned by Dennis, does vary and might even be specifically varied according to the caliber. I have seen no evidence to support this however. Winchester brass is harder than Norma regardless of the caliber. Winchester 300 Win Mag brass will handle high pressure better than Norma 308 Norma Mag brass. This inspite of the fact the 308 Norma was always loaded to higher pressures.
Operating pressures for various cartridges are arrived at by SAAMI based upon what the industry as a whole feels is a safe level for a given cartridge. I'm quite sure the firearms chambered for the cartridge are an important consideration. So, a 30/06 is assigned a lower operating pressure out of deference to the '03 Springfields and '95 Winchesters in which it was chambered. The 270 was assigned a higher operating pressure primarily because Winchester had established it to be OK. The same is true of the 300 WSM. Winchester said these pressures are fine so it is accepted that they are. I suspect there might be some reduction in 300 WSM loadings in the future just as there was with the 300 Win Mag., The 7MM Rem., and the 308 Norma. When factory loads are leaving ejector slot marks on brass and primers are loosening, it's probably too hot!
So, in answer to the original question as to why pressures are different for similar cartridges in the same action; because the assigned working pressure is not necessarily based on the cartridge/rifle combination as tested. It is, instead, based on a combination of factors including established SAAMI working pressures which are , in turn, based on the history of the cartridge. A divergence from this is demonstrated by the published loading data for the 45/70. Although factory loads are loaded to very low pressures, virtually all loading manuals show 2 or 3 levels of data for the 45/70 depending upon the rifle type. Trapdoors are considered weak, Marlins and '86 Winchesters in the middle, and Ruger No. 1's at the top. In this case brass is plainly not a factor BTW. Loading manual publishers or writers are showing an awareness of the widely varying strengths and capabilities of the various firearms in which the 45/70 is chambered. The same thing could be done with other old cartridges but they probably feel it's just best not to confuse the issue.
For what it's worth, I have found Winchester 30/40 Krag brass to be very strong. In a strong action it can be loaded to very high pressures with no loosening of primer pockets etc. Does this mean I can use it in my old Krag rifle and load it to the nuts? Of course not. The Krag is limited by it's action strength or lack thereof.
For rifles which have a strong enough action where the brass can be considered to be the weak link; Gibb's criteria of "good case life with repeated reloading" is probably valid. In other words, if primer pockets don't loosen up after the third loading, pressures are probably OK with the brass/rifle combination in use. If they do loosen up; it's too hot.
In closing just let me say, in classic Gunnutz fashion, You guys are all full of crap! What a bunch of morons! Dintcha ever go to school?!
Regards, Bill.
