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Here is a question I asked Hodgdon, and their answer. Might help answer your question.

Recently I picked up a 3rd edition of the Hodgdon Cowboy Action Data.
As I looked thru the loads, I noticed something that to me is a big question.
I have always heard, never use smokeless powder in black power cartridge guns. Use Black Powder or Pyrodex. Now I see the load data for the .45 Colt.
Using a 250 gr. bullet and 10.5 grs of HS-6 gives a load of 946 fps with a CUP of 13,300.
The Pyrodex load of 37 grs of "P" gives a velocity of 955 fps with a CUP of 13,500.
My question is, since the smokeless loads and the Pyrodex loads are so similar in pressure, how can the Pyrodex loads be less harmful on an old black powder gun than the smokeless loads giving 200 CUP less pressure?
I realize these are test barrel figures but so are factory loads. Please explain this to me.
Their Answer:
On the surface, your logic is accurate. BUT, what does not show on the
surface is how we get to these pressures.

If you look at a tracing of a smokeless powder pressure curve, it will have
a vertical or near vertical left side rising to a pointed peak which falls
away rather slowly on the back side. Kind of like an upside "V" with the
left side squeezed in.

If you look at a blackpowder or Pyrodex pressure curve, it is rounded in a
nice bell curve shape like an upside down "U"

While the two pressure curves get to the same height, they do it in a very
different manner. The time to peak is about 3 times longer for blackpowder
and Pyrodex than for smokeless.

All of this affects firearms in one basic way. The slower the pressure rise
and the longer the time to peak, the less stress on the steel of the
firearm. The steel has time to stretch and contract without tearing.

With fast peak pressures firearms must be designed to contain the stretch
before failure. Sometimes this is done with thicker walls on a barrel/chamber
and sometimes it is done by using different alloys of steel.

I hope that I have answered your question.


Don't draw fire, it annoys people around you.
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