November 30, 2012, 12:25 PM
Join Date: October 9, 1998
Location: Ohio USA
There is no direct correlation between pressure and velocity. Numerous other variables come into play.
My guess is that the precise 10% velocity increase is a coincidence.
Yep - sometimes +P can even be slower than standard pressure.
The old 125 gr NyClad was like that.
Here's a post from a member back in 2000:
Join Date: August 27, 2000
Location: Hudson Valley, NY
Posts: 490 Here is some actual chronograph data, from my testing.
Using a 2" Colt, the standard pressure .38 Special 125 grain Nyclad gave a 10-shot average velocity of 843 fps, for 197 ft-lbs energy. Extreme spread was 84 fps, and Sd was 26.7.
From the same gun, the +P version gave a 10-shot average of 875 fps, for 212 ft-lbs energy. Extreme spread was 60 fps, and Sd was 17.5.
Both tests were run on the same day, with a temperature of 60 degrees.
Regarding the factory specs, remember that the standard pressure listing is from a 2" barrel, and the +P load is from a 4" barrel. The standard pressure load is actually designed to perform from a 2" barrel (and was once even called the "Chief's Special" load), and really performs as advertised.
Pretty ugly eh?