Water capacity (gr)
Fired - 51.6
Fired Resized - 50.4
I think that answers it. Lmk what you think
Your fired vs resized volumes seem typical, and most of the case size reduction is in the neck which gets resized a fair bit more than the shoulder. I have a few measurements from 2 cases each for comparison:
-Hornady fired and resized (0.003 shoulder bump) has a volume around 52.9 grains; this is large due to thin case walls.
-Starline new is 50.85 (I've not measured afterwards)
-Lapua new 50.85, fired 52.15, and 51.50 resized.
So this suggests that your Peterson brass has reduced volume compared to Hornady, Starline, or Lapua.
Plugging your load into GRT:
-2.880" COAL, 40 gr H4350, 50.4 gr H2O case capacity, and adjusted for longer "tikka throat" results in:
2569 FPS at 54,795 PSI.
GRT suggests that your load is safely below peak pressure at an average velocity of 2569 and you are a little higher velocity at 2,595, still within safe range per GRT.
For comparison, GRT calculates my load 2.886 COAL, 42 gr H4350, 51.5 gr H2O at 2,666 fps at 60,641 psi, and my actual velocity is 2,671 (30 round average)- so very close to GRT result. DISCLAIMER- I'm OK with being just above SAAMI pressure.
GRT simulation suggests that you're below pressure and at a very safe load- but there may be intangible factors in your actual gun/load that create the pressure signs you see.
Bumping COAL back to 2.870" to simulate deeper seating gives 2,573 fps at 55,163 PSI. Not as dramatic of a pressure increase as I thought, but that may be because the load is not near peak pressure for the case per GRT. Around 0.050" jump seems to be the most common jump used for 143eldx in 6.5CM and has worked great for me-
no reason to fiddle with your jump based on pressure signs as you're near the optimal seating depth range.
So this leads me to wonder if your die is not fully resizing the case or sizing far enough down the case. What die are you using at what does your resizing procedure entail?
Lastly, if you want to get real tedious to get a more objective and quantitative assessment of pressure, shoot a pressure ladder and measure casehead expansion. For your charge I would do 2x each of 39, 39.5, 40, 40.5, 41, 41.5, and 42gr and measure the casehead diameter with a micrometer (take largest measurement above casehead, average 2 cases per charge).
STOP WHEN YOU SEE EXCESSIVE PRESSURE SIGNS. Plot the expansion (it will be in the 1-2.5 thou range) and look for maximum casehead expansion where the expansion plateaus. You will want to load to below the plateau, which is where the brass is not stressed beyond it's ability to springback. For example, with my brass I see 1.1 thou casehead expansion at 42.2gr, so I load H4350 to 40 gr which gives me around 0.8-.9 thou expansion, indicating that the brass still has springback and has not been pushed into plastic deformation ranges, which correlate with excessive pressures in GRT (greater than 61.5 to >62K PSI).
I have tested casehead expansion up to ludicrous speed and pressure using very slow H4831SC which allows for a very gradual pressure ladder, with the max charge of 47.2gr yielding 2,849 fps at >64K PSI (NOT SAFE, DO NOT REPLICATE) and a plateau around 1.3 thou of casehead expansion above 46gr. Again, no need to run to this extreme with H4350 which is less safe to over-load.
