Barnbwt, are those pressures in actual pounds per square inch (PSI) as measured by strain gauges, or are the copper units of pressure (CUP)?
I as 'cause far too often when CUP systems are used, the results are stated as PSI. There's quite a difference in actual pressure between them in the 50,000 CUP (60,000 PSI) range. Down in the 30,000 range for both, they're virtually identical. SAAMI specs for the .45-70 are 28,000 both CUP and PSI as shown in: