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Originally Posted by FoundFather
I am surprised a .1 reduction in powder will cause such failures when bullet is 15 grains lighter. Especially since with the 165’s I started with the “official” starting load, and with the 180’s I started 5% below the starting load. Does this make sense to you guys?
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It makes perfect sense. Stop and think about it.
When a cartridge fires, the expanding gasses push the bullet down the barrel. Those same gasses also push the slide back, opening the breech and ejecting the empty case. Basic Newtonian Physics: action <==> reaction.
Once the bullet exits the muzzle, the gas pressure in the system drops nearly instantly to zero. Lighter bullets accelerate faster, so for the same powder charge (energy) they spend less time in the barrel.
Example: I opened up the Hodgdon web site and looked at some .40 S&W data for Winchester 231, the powder I use.
For 165 grain bullets, they list two: a Berry's plated flat-point, and a JHP. For the Berry's plated bullet, starting load is 4.7 grains / 25,200 psi. Max load is 5.4 grains / 33,400 psi.
The 165-gr JHP starting load is 4.8 grains / 28,100 psi. Max load is 5.3 grains / 32,500 psi.
They also list two 180-grain bullets, a Berry's plated flat-point and an XTP JHP. For the Berry's plated bullet, starting load is 4.4 grains / 26,400 psi. Max load is 5.1 grains / 33,500 psi.
The 180-gr JHP starting load is 4.1 grains / 23,800 psi. Max load is 5.0 grains / 32,900 psi.
See what's happening? For the heavier projectiles, both the starting and maximum loads are lighter than for the 165-grain projectile, but the pressures (which is what we're concerned with) are in the same ballpark. This is basically because a heavier projectile accelerates slower, resulting in more dwell time in the barrel.
You can run this comparison for any cartridge and any powder. The lighter bullets will call for a higher powder charge, not lower.