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I was playing around with a bunch of different light loads for my wife trying to reach the bottom end of major. I didn't get close with my first attempt, but I thought I'd post the velocity difference with the same powder charge and two different seating depths.

Load was 4.8 grains of W231 using a cast Lee tumble lube 200 grain SWC.

OAL = 1.250" = 686 FPS

OAL = 1.200" = 734 FPS

Obviously there was a pressure increase to match the added velocity.

Just food for thought...
 

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With 5.5/231 and the H&G 200 swc, reducing OAL from 1.26 to 1.25 increased velocity in my 5" barrel from 866 average to 900.
This would suggest that a load with higher pressure to start with may be even more sensitive to seating depth changes.

My experience with hot .38 Super loads would tend to confirm that idea. They were dangerously sensitive to inadvertently shortened rounds.
 

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I have heard that some KBs have resulted from bullets being jammed all the way into the case accidentally before firing.


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It's the physics of nitrocellulose and nitroglycerine powders:

Smokeless powder creates pressure when it is placed under pressure. More pressure generated by the powder, the more pressure is generated by the load. This is why the pressure curve in a load is NOT a straight line. Pressure increases FASTER than the increase in powder.

The case works like a small compression chamber and generates the initial pressure in the load. Smaller chamber, higher pressure.



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