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This is basic information that I should know, but I don't. For example, if I load a .45ACP case with a 200gr lswc, 4.1 grains of Clays, and an OAL of 1.250 would the pressure increase or decrease if I were to shorten the OAL to say 1.225?

Pressures would, indeed, increase. You're burning the same amount of powder in a smaller space, which leads to greater force per square inch.

The amount that pressures would increase is something I don't know. Keep your handloads at or above the minimum overall length, and you'll be fine. The .45 ACP is a "relatively low pressure cartridge" anyway.

Dan

The 9mm and 45acp are fairly forgiving calibers relative to OAL. You can adjust the oal on these without a lot of concern about getting into dangerous pressure spikes. The one you must be very careful about is the 40 S&W. The 40 caliber can have dangerous pressure spikes with even a small amount of decrease in the volume under the bullet.

Remember it is the space under the bullet that determines the pressure. If you are loading the 40 with a different shape bullet than the spec you are using then the oal is not the controlling dimension since the bullets may be different lengths.

If your are not loading to ward the maximums you have some room to work with. I load at, and only at the starting levels so my errors and variations in the cases are not a major concern. The closer to the max you get the greater the need to match the load data perfectly.

I'm wondering, why do you want to shorten the lenght??

Also, remember that when you change overall length, or case length, if you’re trimming, you are affecting volume, not length.

Remember volume involves cubic equations so what would be a tiny change in length, can have a much greater effect than you think. This is why Over all length is in 1/1000 of inches.