I have a question and I hope somone can explain the answer to me! I have been reloading since I was 15 years old (40 years now) and I have noticed but never understood why some powders create more pressure but produce less velocity than other powders. The latest example of this odd situation (odd in my mind anyway) I saw today on the Hodgdon Reloading site. The folks there listed several loads for the 221 Remington Fireball cartridge used in a 24" barreled Rifle.
The listed maximum load using Hodgdon Lil'Gun powder and a 40 grain bullet produced 3,384 F.P.S. at 48,900 C.U.P. then they listed the next fastest load using H4227 powder and its maximum load (using the same bullet of course) at 3,251 F.P.S. and the pressure was 50,200 C.U.P.! I do not understand how or why more pressure in the chamber and barrel would cause in this case 133 F.P.S. less velocity! Again 133 F.P.S. more velocity with 1,300 less C.U.P.! Does anyone understand my question and have an explanation for me?
Thanks in advance.
Hold into the wind
VarmintGuy

Hello Varmintguy,

It has to do with what I call the "area under the pressure curve". Think of a 2-D plot using time as the X-axis, and pressure as the y-axis. Pressure is zero at both ends, and peaks somewhere in between. Some powders have a different "ramp" (or slope) to the peak pressure, while some are more gradual, or smoother, etc. The pressures typically shown in reloading data/manuals are "peak" pressures, or "max Y" on our graph.

So you could get an average pressure during the time the bullet is in the barrel that is higher for a powder with a lower peak pressure, due to the characteristics of the burning curve.

This curve is also what we are optimizing to get peak velocity from a certain cartridge/bullet combination.

Hope this helps, Bill

[ 01-22-2003, 22:46: Message edited by: Bill M ]

Bill,
You are right on. The pressure doesn't relate to velocity. It is IMPULSE that relates to velocity. Pressure times time (with the friction taking away some of the velocity) = velocity. This was something that was dicovered when guncotton was being developed for artillery. Nitroglycerine was thought to be a good "powder" if the right amount was used. What they found was that below a certain point a small charge would not accellerate the bullet to a usable speed and any more would fracture the chamber. The pressure was way too high to allow time to accellerate a bullet. With modern powders the burn rates are modified to affect the time it takes to build pressure in a closed vessel (called a ballistic bomb). A faster powder will build more pressure but for a shorter time than a slow powder. Bullets will respond to the slower powder at a lower peak pressure with more velocity as long as you can get enough powder to build pressure.

PaulS