Twice Barrel wrote: So a 10,000 max pressure measured in CUP in the 1930's is very well likely to be equivalent of 11,500 psi measured by today's standard.
I agree, except as noted: CUP was used for pressuress in the 30-60,000 PSI range and LUP for shotgun cartridges. Here are some better examples of discrepancies in LUP and PSI...
I have several vintages of “Handloader’s Guide For Smokeless Powders” by the IMR Powder Co.
In 1989 all of the shotshell pressures were listed as LUP.
In 1999 all of the shotshell target loads were listed as PSI while the field loads were still LUP.
In 2000 all of the shotshell data was listed as PSI.
By perusing data from 1989 and 2000, I was able to come up with examples of
identical recipes tested by both methods:
In the 20ga Target AA Hull:
1989…W209 primer, 16.0 grains PB, WAA20 wad, 7/8 oz shot, 1180 fps @
10,200 LUP
2000…………………………………………identical components………… ……1195 fps @
11,700 PSI
In the 12ga Target AA Hull:
1989…W209 primer, 19.0 grains 700X, WAA12 wad, 1 1/8 oz, 1200 fps @
9200 LUP
2000…………………………………………identical components……………… 1190 fps @
10,800 PSI
And my favorite, in the 16ga (Win AA type) Compression Formed Hull:
1989…W209 primer, 20.0 grains 800X, SP16 wad, 1 1/8 oz, 1175 fps @
8800 LUP
2000…………………………………………identical components……………… 1210 fps @
11,000 PSI
For the identical loads, the “pressure number” increased by 14.7%, 17.4% and 25% respectively.
A perusal of the published literature will reveal that chamber pressures measured in LUP, and erroneously reported as PSI, will consistently err on the low side. The above published data are examples of that. Furthermore, it should be clear from just the 3 examples cited that a 1920's era maximum service pressure of 9500LUP could actually equate to 10,897 - 11,875 PSI peak chamber pressure.
So, did SAAMI really increase the recommended service pressure when they upped it to 11,500 PSI? or did they just discover that the 9500 LUP number was really more or less equivalent to 11,500 PSI? Until I hear otherwise from a professional ballistician, I'll believe so.
Beware the man with one gun...he likely will bore you to death in others ways, too.