Thread: Seating depth question

I loaded up virgin brass with a BTO measurement of 2.502in to be 10thou off.
The once fired case shoulder moved forward 5 thou, I've full length sized to bump shoulder back 2 thou so my question is.....to maintain the 10 thou jump, I seat the projectile 3 thou deeper to shorten the BTO to 2.499.....does this sound right?

Changes in shoulder length won’t change your bullet seating depth. CBTO will be constant from base to ogive if you don’t adjust your seating die


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I'm interested to see what the experts say, but it doesn't sound right to me. The distance from bolt face to lands hasn't changed, so your BTO measurement shouldn't change. The case is longer because your chamber is longer than your virgin brass, but the bullet has left the case before that happens.

Cheers.....I'll leave the seating die where it is.

If you watch any of Erik Cortinas videos, he will say to adjust your seating depth. The firing pin supposedly smacks the case forward as far as it will go before denting in and setting off the primer, with the brass then stretching backwards to the bolt face.

So having 5 thou headspace on virgin brass with 10 thou jump for the bullet means only 5 thou jump once the case moves forward. Same BTO measurement with only 2 thou headspace on resized brass would mean 8 thou jump.

Whether or not that's significant probably depends on how finicky your chosen bullet is.

Originally Posted by Pommy

If you watch any of Erik Cortinas videos, he will say to adjust your seating depth. The firing pin supposedly smacks the case forward as far as it will go before denting in and setting off the primer, with the brass then stretching backwards to the bolt face.

So having 5 thou headspace on virgin brass with 10 thou jump for the bullet means only 5 thou jump once the case moves forward. Same BTO measurement with only 2 thou headspace on resized brass would mean 8 thou jump.

Whether or not that's significant probably depends on how finicky your chosen bullet is.

Interesting, never thought of it that way!
Makes sense though


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Cheers @Pommy yeh been watching a few of his vids but havnt seen that particular one.
So on that basis seating 2 thou deeper will compensate for the 2 thou of headspace and maintain a 10 thou jump - yes?

Originally Posted by wsm junkie

Cheers @Pommy yeh been watching a few of his vids but havnt seen that particular one.
So on that basis seating 2 thou deeper will compensate for the 2 thou of headspace and maintain a 10 thou jump - yes?

By my math, you had 5 thou jump last time because the headspace took up 5 thou jump of the 10 thou you got from your BTO measurement. So you'd want to seat 3 thou further out now because the case will be taking up 3 thou less.

Or... Don't. Just leave the die alone, go shoot another group with your resized brass and see if you can notice any difference at all before making any changes.

I don't adjust dies myself as I'm not a consistently good enough shooter to tell the difference, and I'm usually shooting fairly forgiving bullets with much more that 0.020" jump anyway.

I'm not entirely convinced by the theory either. Seems logical but perhaps oversimplified. Is the primer so strong that the inertia of the round is overcome and the whole thing moved forwards before the primer goes off? Maybe with a little 222 but what about a much heavier 300 win mag? What if you have very light neck tension, would the bullet stay put and the case move up around it?