Seating depth tests & 1x fired vs new brass

[Sheldon N]

Just thinking out loud here so bear with me... working on developing a new load for my 308 and this is the first time I've really gone through the process.

Initial load testing for charge weights done with new Lapua brass. Seated 20 thousands off lands per my Hornday comparator tool. Ready to move on to seating depth testing. But here's where I'm getting confused...

New Lapua brass measures roughly 1.619 to 1.620 from base to shoulder. My Hornady 308 case for the comparator tool measures 1.6195 to the shoulder, so that's consistent. However, after firing my brass measures 1.625 to the shoulder, so I've got between 5 and 6 thousandths of headspace on new brass. I'm thinking that also means that my base to ogive and OAL measurements are going to vary by the same amount between new brass and 1x fired brass? (not to mention neck tension differences since I'm not expanding/resizing the new brass).

Will this mess with the results of any seating depth test I do, if I use new brass? The load data thread suggested seating depth increments of 0.003 when testing, so that could have my results be off by two full steps between new and 1x fired brass.

Am I over thinking this? Is the safe answer only to do seating depth tests with your 1x fired brass and not new brass, or does it not make a difference?

 

[watercam]

Your shoulder set back does not affect your OAL (case base to bullet ogive). Adjust your SIZING die so that your shoulder is pushed back .001 - .002". The measurement of your base to ogive is independant of your sizing adjustments. Use the Hornady OAL length tool (measure three times) to find your case base to ogive and make the adjustments with your SEATING die for your jump test. I pick a safe, non-maximum charge (1 gr. under max. for example) and jump conventional ogive shape bullets .040", .030", .025", .020" and .010" off the lands. If you want to jam as well have at it. For VLD bullets I start at .030" and go until just touching the lands. For solid copper bullets, try .080", .050" and .030" jumped to start. Assuming you have done a powder charge test, try the best charge test with the best jump test and see how that goes. Tweek at will.

 

[Sheldon N]

But isn't my comparator tool going to give me a measurement that is 6 thousandths shorter than the actual depth to the lands? When I shove it in by hand, it's resting on the shoulder of the case against the chamber, correct? If so, there is 6 thousands less case body extending backwards to the base vs a fired case, and that all gets added into the total measurement of base to ogive.

So if I load a fireformed round to the same base to ogive measurement, it will also headspace off the shoulder of the case and as a result I think the bullet will be sitting 6 thousandths farther back from the lands. I think the case body is going to be a snug fit between bolt head and shoulder and a longer portion of the OAL to ogive measurement, correct? Since the base to ogive is the same, the extra case depth gets subtracted from the shoulder to ogive distance because the base to shoulder is longer. (Yes, my FL sizing die is adjusted to bump shoulder 1 or 2 thousandths)

I guess part of this I'm assuming that what happens when the gun is fired is that the bullet starts its travel forward from the position it's sitting when the case is resting against the shoulder of the chamber (held in place by ejector spring and/or pushed forward by firing pin), and that the brass case will expand/travel backwards on firing to fully contact the bolt and expand to fill the chamber. So where the bullet sits on chambering is what really matters.