Seating depth for cast bullet ammo for Bolt action Rifle:
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I want to seat a cast bullet so the nose touches the lands. I will make up several dummy rounds at different COAL to determine the Best COAL for a given rifle/bullet combo, judged from witness marks on the nose. I will stick with that COAL and then work on other factors if I don't get the desired accuracy.
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That works great for bullets that aren't PC coated. PC adds diameter to the nose, which will change when it engages the lands...changing the best COAL for your rifle/bullet combo, and if the PC isn't fairly uniform, you could have some variance with launch pressures.
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This all goes to the theory that the best COAL of cast bullet rifle ammo is where bullet touches lands. This may or may not be true, but it's where I've had the best accuracy for typical cast bullet loads. I believe it's the best possible chance for a square launch and uniform launch to launch pressure, for a beginner to loading cast bullet rifle ammo. Once you have some success at this, and want to experiment for whatever reason, then try a shorter COAL to see if a Jump will help or hurt accuracy.