Linear correlation between case capacity and pressure/velocity?

[canezach]

Greetings, gents! I realize if I take two cases of the same caliber and one has smaller case capacity, and all other factors remain equal (charge weight, seating depth, neck tension, etc), the smaller case capacity will produce higher pressure. My question is if I reduce the powder charge by the same percentage, is the result a linear reduction in pressure?

For example: Hornady 6.5 Creedmoor brass has, let's say, 52 grain of H2O capacity. My load is 42.0 grains of powder. It produces a MV of 2840 fps. Now let's say I use a piece of Alpha brass. It has a case capacity of 50 grains, or 3.84% less. If I adjust my charge from 42 grains to 40.38 grains (same 3.84% reduction), will my pressure and velocities be the same, or is there something I'm not accounting for?

 

[STI_1911_Guy]

I dont believe there will be a linear correlation between those two separate sets of variables. Given a perfect world where everything else remained the same mainly neck tension like you said i believe the change in a 3.84% reduction in powder will have a greater effect than the case volume change of 3.84% will. The powder generates much more or less energy with a 3% reduction/increase than the same % of case volume change has on pressure. I dont have data to back this up nor did i actually think back to any of my teachings(pressure dynamics) on this, this is just merely my thoughts at a quick glance, take it for what its worth.

 

[Milo 2.5]

Linear is not a word I'd use either, if you had Quickload, with the case capacity feature, you theoretically could duplicate the important paramaters. Pressures, barrel time, velocity.
I guess I could help out if you give me barrel length, powder, oal of the rd, tip to base, bullet used etc...
Just subtracting a pct doesn't with brass and powder because nothing can be linear.
Just subtracting a pct can't work with brass and powder, it can't be linear.

 

[Rerun7]

Interesting thought and at first glance it would seem feasible but my gut tells me it won't play out that way. If you continue to extrapolate that methodology to a higher decrease, say 50% I would not expect it to be true. I would assume that grain weight of the bullet would also have to come into play at some point. However with something like a 3% reduction the variance may be close enough to be ok in a real world scenario depending on the type FPS difference you expect. Would have to run some tests on a chrono to be sure but I'm guessing your FPS is going to fall off by reducing 1.6gr of powder even with the smaller case.

 

[morganlamprecht]

if i remember i can look at my load data and give you numbers on it...

i run different types of brass in my 308 and 6mm calibers with bullets loaded to the same speed so it doesnt affect dope

 

[AIAW]

Remember there are lots of variables that are changing here - not just the powder content.

A) Oxygen in the case - more if there is less powder. Granted, this is minimal since the powder contains it's own oxidizer, but still it's there. Air also pads the pressure wave at ignition.

B) Total powder that must combust given the primer's ability over time. Less charge, possibly more lag before core powder ignition occurs. Last item is a very small variable, but it's there.

C) The flash-rate of granules that may be closely packed versus loosely.

D) Peak pressure timing. Less time = more pressure


Here is an example that I threw together in QuickLoad - using my 260 as an example:


Standard case and charge. (Larger version)
[IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/image.ibb.co\/dXWQfa\/260_Standard.jpg"}[/IMG2]




ONLY a case-capacity reduction of 1 grain. (Larger version)
[IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/image.ibb.co\/m9OStv\/260_Smaller_Case.jpg"}[/IMG2]




Now, a 1g reduced charge. (Larger version)
[IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/image.ibb.co\/grdStv\/260_Smaller_Case_Reduced_Charge.jpg"}[/IMG2]

 

[Sheldon N]

Greetings, gents! I realize if I take two cases of the same caliber and one has smaller case capacity, and all other factors remain equal (charge weight, seating depth, neck tension, etc), the smaller case capacity will produce higher pressure. My question is if I reduce the powder charge by the same percentage, is the result a linear reduction in pressure?

For example: Hornady 6.5 Creedmoor brass has, let's say, 52 grain of H2O capacity. My load is 42.0 grains of powder. It produces a MV of 2840 fps. Now let's say I use a piece of Alpha brass. It has a case capacity of 50 grains, or 3.84% less. If I adjust my charge from 42 grains to 40.38 grains (same 3.84% reduction), will my pressure and velocities be the same, or is there something I'm not accounting for?

According to Quickoad modeling, using a similar percentage adjustment of powder vs capacity is not a bad approach to match peak pressure. However even if you match peak pressure, velocity and barrel time can be dramatically different and that can affect whether you stay in the node from your prior load data.

Biggest thing that you're not accounting for is the effects of volume and time. Velocity and barrel time are affected by the rate of acceleration of the bullet, what position in the barrel it is at when the case and chamber reach max pressure, and the total volume of pressure behind the bullet at the moment it reach that max pressure. All those interplay in nonlinear fashion to produce different velocity and barrel dwell time, even if you match the peak pressures.

 

[Milo 2.5]

I tried to edit my 1sr post, can't, with powder, brass and bullets, nothing is linear. Give me your bullet, oal tip to base. powder used, and I'll crunch some numbers in QL, get you within maybe .2 gr of a charge.
I did this in a 6.5 creed with RL17 & RL23 powders, the only thing that was skewed was a heavily compressed charge of 23 that altered my seat depth by .010". I shot 3 shot groups at 500 with identical result as far as group size, POI changed to 4" low and right was all.

 

[canezach]

Guys, this is why I love the Hide! My question arose because I've been using Hornady brass forever in 6.5 Creedmoor. I have about 500 pieces of 1x fired Prime brass and 200 pieces of new Alpha brass that I haven't developed a load for. My plan was to start all over in load development with the Alpha brass as soon as my new DTA barrel arrives, but in order to start low, I was going to reduce the low charge by the same percentage as the case capacity is reduced. With that said, I don't want to run into an issue where I'm throwing an underpowered load either.

 

[morganlamprecht]

Here's my numbers to do with as you want

6 Comp Match/115 dtac

Norma Brass Capacity: 54.9 avg
Charge: 46.3 gr h1000
velocity: 2950 fps

Winchester Brass Capacity: 55.8 avg
Charge: 47.0 gr h1000
Velocity: 2940 fps

bullet seated the same, chrono'd on the same day...one difference was brass type and charge weight

 

[canezach]

Guys, I really appreciate all the replies. Milo, I'll take you up on that QL comparison. As soon as I get home, I'll measure some loaded rounds and gather some other data, like case capacities of Hornady and Alpha brass.