help me understand pressures.

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PT1911

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I dont claim to know a lot about reloading as I am just breaking the threshold. That said, I pride myself on having at least some bit of sense and thinking about pressures is just plain frustrating. SO, to the root of my frustration... How can one push similar bullet weights at similar speeds and have one be lower pressure than the other?

Case in point, the 44 mag v the 45 colt...

45 colt buffalo bore 325 gr at 1325

44 mag 320 grain at 1300 fps

Now i am not saying that these are popular loads, that is secondary. My issue comes with the "popular" knowledge that the 45 colt is a low pressure round whereas the 44 mag is a high pressure round...

I'm confused!!!
 
The main issue here is that the .45 Colt is a low pressure round only when it's a low pressure round. When loaded hot (for guns like the Ruger Blackhawk) it's a high pressure round.
 
As a fellow beginner that want to think that he knows some of what he's read I'd say what you have here are two rounds that are close to the same muzzle energy. At first glance you would expect that the max pressure of the loads would be equally as similar. But you need to stop and look at the case volume as well. The bullets are going to be very close to the same depth in the cases but the cases are somewhat different.

Caliber....... Case neck...... case length
.44Mag...... .429 ............. 1.285
.45Colt ..... .480 ............. 1.285

So the .45 has a case volume, assuming similar bullet seating depths of .480/.429 times as much or 10 to 11% more volume. This extra volume will keep the peak pressure slightly lower for the .45Colt for the loads you used as examples.

I recently reloaded big batches of both .38Spl and 9mm. In this case I used 124 gn bullets for the 9mm and 125gn bullets for the .38Spl so you can say they are the same. For similar velocities (downloaded 9mm to be used for IPSC) the .38Spl had just over half the peak pressure of the 9mm. Of course the same 3.6 grains of the powder used for both loads all but filled the 9mm to brimming while the same amount in the .38Spl filled about a third of the availabel volume. It is this extra volume that kept the peak pressure lower.

Also keep in mind that the peak pressure is just that, a peak. As the burn starts up the bullet begins to move. At some point the burn is far enough along and the bullet moved little enough that the pressure will spike to its peak. But as the bullet continues down the barrel the powder finishes the majority of it's burning and the volume is increasing due to the bullet moving along the barrel. So the pressure only peaks for a very short time and then begins to fall away until the bullet leaves the muzzle where the pressure drops more quickly.

Howzatt?
 
The 45 Colt was originally developed in the days of black powder. Lots of powder, low pressure, modest performance. Current manufactured 45 Colt ammunition is loaded to pressures safe for those old guns and approximately match the black powder load performance.

For years, folks have been loading 45 Colt to 44 Magnum levels for their Ruger Blackhawks and T/C Contenders. These handguns are designed for 44 Magnum pressure levels. Many reloading sources have Blackhawk/Contender data sections.

Don't ever shoot a supercharged 45 Colt load in a standard 45 Colt gun such as a Colt SAA.
 
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