Peterson pressure 6.5 cm

[Andouille]

You aren't missing anything on the pressure thing....Deeper means more pressure, which means backing off powder more.

I would load 5 or 10 more at 40gr btw with only 1 being spicy and test....or...drop to 39.5gr...or....if you want a bit more gas out of it, N555, RL16 and staball 6.5 are options that are faster for testing.

I agree, a little slow, but not crazy. H4350 has been the best powder for my Tikka 6.5CM, but I've had similar velocities (+/- 25 fps) and respectable accuracy with H100V, Staball 6.5, Superformance, and H4831SC. N555 showed pressure at 2566 fps so I didn't pursue it further.

 

[wind gypsy]

Very interesting- I still don't get how reducing case volume substantially by seating deeper wouldn't affect pressure, much in the same way changing brass volume affects velocity. I'm short on time now but will run GRT in a week or so and plot the results on a similar graph. The caveat is that I'm unsure if GRT will model a jammed bullet, so the output may be more linear in the longer COAL end of the chart.

Why does a jammed bullet have more pressure?

I think with more jump, there's more gas/pressure that escapes around the bullet prior to it obturating to fill the bore. That study with a round nose bullet might have shown a larger jump before hitting min pressure than a modern spitzer, but i'm not positive.

 

[wind gypsy]

I agree, a little slow, but not crazy. H4350 has been the best powder for my Tikka 6.5CM, but I've had similar velocities (+/- 25 fps) and respectable accuracy with H100V, Staball 6.5, Superformance, and H4831SC. N555 showed pressure at 2566 fps so I didn't pursue it further.

N555 didn't seem to be an upgrade on velocity prior to pressure in my 6.5 either.

 

[streamerfish]

N555 didn't seem to be an upgrade on velocity prior to pressure in my 6.5 either.

Yeah...my experience is a bit more but not alot more on N555....Could be a small enough difference to get lost in the noise so to speak.

 

[il_cop]

@Andouille thank you for the write-up, I appreciate it.

In terms of resizing, nothing special:
Lee Ultimate
FL Sizer w/o expander ball
Touch shell holder + 1/4 turn
Lube and size
Sinclair expander mandrel at .262

This is the same setup I've used on my other 6.5 CM but with Lapua LRP brass.
That rifle (savage) was right at 2700 with 143 eldx, 22" barrel, 41 gr RL16
This is my first time using Peterson

 

[pods8 (Rugged Stitching)]

Just as a sanity check question, is all the sizing lube cleaned off your brass?

 

[il_cop]

@pods8 (Rugged Stitching)
I appreciate it. yes

 

[Andouille]

To say seating shorter or longer increases or decreases pressure unequivocally would be wrong. I assume it's dependent on bullet, powder, case fill, etc. From the study/graph below, lowest pressure was jumping 0.250" to lands. Be interested in what GRT has to say about this specific use case.

True, I should have qualified my statement that increasing seating depth when a resonable distance from the lands generally results in a pressure increase as useable case volume decreases- this would be the near the low point and rightwards on the curve you posted. This is consistent with statements by Brian Litz and Jeff Siewert summarized below.

Why does a jammed bullet have more pressure?

I think with more jump, there's more gas/pressure that escapes around the bullet prior to it obturating to fill the bore. That study with a round nose bullet might have shown a larger jump before hitting min pressure than a modern spitzer, but i'm not positive.

Because pressure builds very quickly when there is a temporarily fixed combustion volume when the bullet doesn't have any room to accelerate (i.e., get a running start) before hitting the lands and beginning engraving.

The text below synthesizes published work by Bryan Litz and Jeff Siewert on how seating depth, bullet length, and bullet design influence chamber pressure. Yes, I had a little help from AI, but everything is referenced and lines up with Jeff's book. The explanation below generally explains the pressure curve you posted earlier. Quick summary:

• Peak pressure varies non-linearly with seating depth; pressure is elevated at or near the lands, reduced at modest jump, and rises again with deep seating due to reduced case volume (Litz).
• Seating longer does not guarantee lower pressure; near the lands, delayed bullet motion and early engraving can increase peak pressure even when usable case volume increases (Litz).
• Bullet release from the case neck requires relatively little force compared to engraving; neck tension is not the dominant driver of peak pressure (Siewert).
• The timing of bullet motion and engraving strongly shapes the pressure–time curve; models that ignore free-run and engraving effects under-predict pressure near the lands (Siewert).
• Bullet length influences pressure indirectly through seating depth (case volume), engraving resistance, and sensitivity to lands interaction (Litz, Siewert).

More detailed summary:
A. Core Points from Bryan Litz (COAL/CBTO and Pressure)- Pressure vs. seating depth is non-linear: for a fixed charge, moving from jam to a modest jump typically reduces peak pressure, but seating deeper can raise pressure again as case volume decreases. Seating a bullet to touch or jam the lands can noticeably elevate pressure relative to being a few thousandths off the rifling due to immediate resistance to initial bullet motion. Loading longer generally increases available internal volume and can reduce pressure/velocity for a given charge — until lands contact effects dominate near zero jump.

B. Core Points from Jeff Siewert (Shot-Start, Engraving, Interior Ballistics) The pressure/force required to move a bullet from the case neck (“shot start” in the case mouth) is small compared to engraving forces; therefore, neck release is not the dominant contributor to peak pressure near the lands. Interior ballistic behavior depends strongly on the timing of bullet motion and engraving; delayed motion (e.g., at/near jam) alters the pressure-time history in ways that simple volume-only reasoning does not capture. Modeling approaches benefit from explicit representation of free-run, engraving, and resistance terms rather than assuming a purely volume-driven pressure response.

C. Integrated Interpretation (Litz + Siewert)- Taken together, Litz’s COAL/CBTO discussion and Siewert’s interior ballistics framing indicate two competing mechanisms control peak pressure when seating depth changes: (1) effective initial combustion volume (deep seating increases pressure), and (2) early-time resistance and the timing of bullet motion/engraving (near-lands seating can increase pressure even when volume increases). This produces a qualitative “U-shaped” peak-pressure trend vs. CBTO/jump: high at jam, lower at modest jump, and rising again at deeper seating.

REFERENCES:
Bryan Litz – Effects of Cartridge OAL (COAL) and CBTO https://bergerbullets.com/wp-content/uploads/2013/03/COAL.pdf
and
Jeff Siewert – Measurement of Bullet Seating Force & Effect on Interior Ballistics Performance (interesting read regarding annealing, engraving force, etc)
https://storage.googleapis.com/wzukusers/user-33607572/documents/bd070255949a4c79be8c8add336f764a/Measurement of Bullet Seating Force.pdf

 

[Andouille]

@Andouille thank you for the write-up, I appreciate it.

In terms of resizing, nothing special:
Lee Ultimate
FL Sizer w/o expander ball
Touch shell holder + 1/4 turn
Lube and size
Sinclair expander mandrel at .262

This is the same setup I've used on my other 6.5 CM but with Lapua LRP brass.
That rifle (savage) was right at 2700 with 143 eldx, 22" barrel, 41 gr RL16
This is my first time using Peterson

Seems like a good resizing order of operations, nothing weird unless you're bumping back some crazy amount (>>>10 thou).

I figure (based on a lot of googling) that 6.5 CM loses about 18-22 fps per inch of barrel (with increasing loss as barrel shortens), so your 22" barrel is at a normal velocity for 41 gr of RL16.