edp2k
I also find it very interesting that the velocity and pressure given for loads for the 2 "Lead Semi-WadCutter" bullets
varies significantly between the 2 bullets.
The 2nd bullet, "MEI" is a commercial cast Meister brand bullet.
I don't know what the brand/style the 1st bullet is or how it differs from the 2nd bullet.
An excerpt from that website's data for CFE Pistol powder:
158 GR. CAST LSWC ___ Hodgdon CFE Pistol 5.3 gr. 1,048 FPS 16,100 PSI
158 GR. MEI CAST LSWC Hodgdon CFE Pistol 5.0 gr. 1,029 FPS 16,700 PSI
Why does 0.3 gr more powder produce 600 PSI LESS?
It shows that different instances of guns (even of the same make and model) and
supposedly the "same" bullet (158 gr lead) can make a significant difference in velocity and pressure.
A variance of 19 fps and 600 psi is really not a "significant difference". You can easily have that much variance in fps and psi in back to back tests of the exact same ammunition in the same test fixture/firearm. Also the use of 2 different bullets even of the same weight can easily cause more variance than that. The variables of different lube, different alloy, different bearing surface, different seating depth along with others cause the difference.
Anytime you see a pressure figure quoted you must understand there are +/- pressures also and what you see posted is the "average". It is the same for velocities posted. There is always a +/- or ES (Extreme Spread) associated with the average figure. SAAMI has allowable +/- ranges (it's complicated btw so don't ask for a set number).