The two parties agreed on one particular bullet, the 150 grain, Nosler Sporting Handgun. According to accounts on the internet, this bullet is expands very easily so there is no need to load it to maximum velocities to get reliable terminal performance. This bullet has the same sectional density as a 115 grain 9mm bullet.
The puzzle is that the loading information is all over the place. My plan is to start low and to use the chrony to dial into a specific velocity. The reasons follow.
Scary internet stories
One quickly encounters many scary stories on the internet regarding reloading the 40 S&W. There are several factors that come into play. Primarily, the earliest generation of 40 S&W handguns were often reworks of 9mm designs. The designers found that they had to shave away portions of the chamber to facilitate reliable feeding. This is sometimes called "ramping". The removal of some of the chamber resulted in portions of the case being "less supported".
|This happened to be a 45 ACP. It shows unsupported case wall below (in this orientation) the extraction groove. Case on the right had a blow-out. Photo from HERE|
Most of the time the brass was up to the task. Sometimes...not.
Another factor is not visible. The extraction groove on the 40 S&W is larger than the groove on the 9mm Parabellum. The top of the groove is 3.5mm (SAAMI)-to-4.0mm (measured) above the base of the case. Meanwhile, the 9mm Parabellum has a skimpier extraction groove that only goes 2.7mm (SAAMI)-to-3.3mm (measured).
The trend is reversed inside the case. The web of the 40 S&W measures 4.3mm deep while the web of the 9mm case is 4.5mm deep. In short, the web of the 40 ends at the top of the extraction groove while the web of the 9mm Parabellum extends 1.2mm above the groove. The 9mm simply has more brass in the most highly stressed regions at the base of the case.
|High speed photograph of unsupported 40 S&W case walls bulging to contain peak pressure via "membrane" stresses.|
The bottom line is that there is less meat there in the 40 S&W to save the inattentive reloader from mistakes.
Wide disagreement on data
Reloaders are encouraged to consult many sources of data.
The 40 S&W is a great example of why this is a wise practice.
|The circled data point is from the powder manufacture for the powder I have in-hand for a 155 grain bullet.|
No data is available for the powder I have in-hand for the bullet I will be using. The data point that is circled in red is of a heavier bullet and the powder I have in-hand. This data was published by the manufacturer of the powder. Conventional wisdom suggests I should be "safe" using the 8.0 grain powder charge with the lighter bullet. Looking at the data, the 8.0gr powder charge looks like the flier. In fact it looks like it will be hotter than a popcorn fart.
That is why I am going to start low (6.0 grains or so) and work my way up to a velocity that is well within the envelope for the cartridge/bullet weight/burn rate combination. And then I will stop. Right now I think stopping at 1100fps sounds safe and practical.
|"Adiabatic" means that no heat transfers in or out of the control volume. K, the ratio of specific heats assumed to be 1.3|
Velocity and peak pressure are correlated although the relationship is not linear. If there are no other significant changes, then a 10% increase in velocity (1210fps instead of 1100fps) is the result of a 21% increase in peak pressure. So the load that should be producing 32ksi is now knocking out +39ksi.
The correlation between velocity and pressure means the reloader can sometimes work backwards. I do not have a way to directly measure pressure but I have a chrony and can measure velocity. Regular folks call it "anal". Reloaders call it "prudence". Attorneys call it "due diligence". It is my intention to never need an attorney. Even more, I never want to explain to a "mom" how Billie shot his eye out.