Extrapolating Charge Weight

That's interpolation (between), not extrapolation (beyond).

Yes, many of us do it all the time. Yes, physics and chemistry applies. Yes, you can blow yourself up if you do it wrong. Yes, you need to know what normal and high pressure looks like with your primers in your gun.

Starting with the starting charge for those two recipes, figure out the difference in charge weight and initial volume (case volume under the bullet). Interpolate charge weight via bullet weight, reduce on a % charge basis to account for any difference in volume, and there's you're starting charge.

If you were using a powder that was friendly to low charges, you'd reduce further to start. Needless to say, load one or three to try before loading a box full.

Queue scandalized protestations of mortal danger in 3. . . oh wait, this is really quite a bit safer than building your own dirt-track go cart, making your own hobby rocket motors because Estes' aren't big enough, or grinding your own blackpowder for homemade fireworks. Carry on, and don't make an ash of yourself.

I would graph as many loads around what you have and look for the best middle. H110 is not one I would be excited about doing this with because of how it acts. I do this with cast loads with unique and 2400 all the time. My moulds never match a book load.

Load 1 each at 35 grs , 36, 37, 38. with your 460 gr. Watch for pressure nearing 38 grs. The powder is going to burn just fine at 40,000 PSI.

Just what i would do.

In the Field Artillery (Substantially Bigger). I have done it, with desired results. I would suggest to go thru the process, then reduce by 10%.

Good Luck

Dan
Retired Field Artillery Automated Fire Control Operator & Instructor.

As others mentioned above, but reiterating since we’re talking about a relatively dangerous practice of going outside of book data:

Interpolating data can work, but only when relevant guideposts are used. In this case, you’re trying to determine a load for a lead intermediate weight but have reviewed a lead lighter and a jacketed heavier. Bad guideposts, because the jacketed data isn’t truly applicable.

Second, I’ll say that it is exceptionally rare to find something truly unknown under the sun. Someone out there has loaded this bullet or similar in this cartridge, so it’s just a matter of diligence to find that data and experience. Powder and bullet manufacturers often have data which may not be published in their manuals, but remains in their database.

Personally, when I play with this type of data, in the absence of Quickload data, it is one rare instance where I use kinetic energy as a guidepost. Kinetic energy developed at the muzzle isn’t perfectly related to potential energy dumped into the case (powder charge), but obviously, the KE out can’t exceed the PE input. I’ll review relative KE per grain input for lighter and heavier bullets and attempt to interpolate a relative scaling for charge weight, more consistent than simply “splitting the difference” (linear interpolation) between a heavier and lighter bullet. Similarly, I’ll occasionally also plot a curve of min and max charge weights vs bullet weight to allow a non-linear interpolation along the curve, rather than simple linear interpolation. Some challenges for all of this, of course: the availability of data for truly representative bullets of various weights, unknown pressure standards for various loads (some might be max pressure, some might be max velocity for a bullet design but lower pressure than SAAMI max. Thirdly, I’ll also review case fill ratio for H110/W296 using the relative seated shank lengths and case volumes as references.

Given those 3 curves (KE/grn, grn powder/grn bullet, and fill ratio/bullet weight), plus any located user data or manufacturer data, I will divine a bracket for starting and max load to use for my first round of testing.

your book data is out of the hodgdon annual manual (last page) i'm presuming. to extrapolate from the 440 grain lead bullet load you show to the 460 grain bullet load, i would compare the 385 grain rem hp load (lil'gun) to the 400 grain sierra jsp load (also lil'gun). the bullet weight difference is 15 here which is close to your desired difference of 20, and to your total weight (400 here/460 desired). also we are comparing apples-to-apples as these two loads are jacketed.

max powder amount for the 400 grain bullet load is 2.5 grains less than the 385 bullet load. the max pressure for the 385 grain bullet load is 2400 psi less than for the 400 grain load, so the powder amount spread here would be a bit more if the pressures were equal, maybe 3.5 instead of 2.5.

so, applying the 3.5 grain spread to your 440/460 grain scenario, i would start 3.5 grains below the starting load you listed for the 440 grain bullet (30.5 grains).

luck,

murf

Interpolation or extrapolation can be helpful, both methods are only mathematically accurate when dealing with straight lines. Powder seems to burn more efficiently at higher loads, therefore it's plotted as a curve. So these methods are best used when the changes from published data are small.

rfwobbly said:

Interpolation or extrapolation can be helpful, both methods are only mathematically accurate when dealing with straight lines. Powder seems to burn more efficiently at higher loads, therefore it's plotted as a curve. So these methods are best used when the changes from published data are small.

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What is plotted as a curve? Pressure? Velocity? Examples (measured, not estimated)?

Find someone with Quickload. Replicated the published load data in Quickload to ensure you have things setup right in QL and then you can substitute your new bullet and work up a good safe starting load. Chronograph that and refine QL a bit more and then go to town working up to your desired velocity. I have a copy but my new computer does not have an optical drive yet to install it or I would give it a go.

fxvr5 said:

Pressure?

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isn't pressure a square function?

murf

Interpolation and extrapolation are not limited to linear relationships. Interpolations and extrapolations of non-linear functions are not so different than those for linear systems. A person simply needs to sufficiently define the relationship to fit the non-linear regression, and interpolate or extrapolate from the resulting curve. Interpolation, in that case, is much easier than extrapolation, as the curve can be graphically defined without a mathematical regression, but either inter or extra work fine.

fxvr5 said:

What is plotted as a curve? Pressure? Velocity?

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Pressure vs. OAL, Pressure vs. Load, Pressure vs. Velocity, Pressure vs. anything a reloader might need to extrapolate.

rfwobbly said:

Interpolation or extrapolation can be helpful, both methods are only mathematically accurate when dealing with straight lines.

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Interpolation isn't necessarily linear.

rfwobbly said:

Powder seems to burn more efficiently at higher loads, therefore it's plotted as a curve. So these methods are best used when the changes from published data are small.

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Yes and yes!

Modern powders are progressive (they burn faster at higher temperature/pressure). That'll bite you hard across large changes, especially extrapolations.

But across small increments, linear is close enough!

rfwobbly said:

Pressure vs. OAL, Pressure vs. Load, Pressure vs. Velocity, Pressure vs. anything a reloader might need to extrapolate.

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Pressure versus OAL, and pressure versus load for (some) handgun powders, in the link below are distinctly linear. See page 3 for charts.

http://www.castpics.net/LoadData/Freebies/RM/Ramshot/Ramshot_3.pdf

Once again some forum experts' claims appear to be disproven by facts.

My suggestion is that when someone makes claims as if they are fact, ask them to show evidence supporting their claim. If they can't provide that evidence, there is no reason to believe it is true.

Hitchens's razor applies here. "What can be asserted without evidence can also be dismissed without evidence."

Funny stuff!

edwardware said:

Queue scandalized protestations of mortal danger in 3. . . oh wait, this is really quite a bit safer than building your own dirt-track go cart, making your own hobby rocket motors because Estes' aren't big enough, or grinding your own blackpowder for homemade fireworks. Carry on, and don't make an ash of yourself.

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Like!

fxvr5 said:

Hitchens's razor applies here. "What can be asserted without evidence can also be dismissed without evidence."

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So what approach to charge wt. did you decide to use for the ladder test, .40?

forty_caliber said:

Seems like a ladder test might be the best approach to the problem.

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rfwobbly said:

Interpolation or extrapolation can be helpful, both methods are only mathematically accurate when dealing with straight lines. Powder seems to burn more efficiently at higher loads, therefore it's plotted as a curve. So these methods are best used when the changes from published data are small.

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Only true for 2 point interpolation and 2 point extrapolation. And, for exponential functions, graph paper of logarithmic or semi logarithmic design (as appropriate) can be used to make an mathematically exponential extrapolation graphically appear as a straight line although the mathematical function is undeniably non-linear.

Extrapolation is always going from a known / measured to the unknown / unmeasured, while interpolation is bounded by knowns into unknown / unmeasured. Both involve risk.

I agree.

Walkalong said:

You have a 460 Gr lead gas checked bullet, and good data for a very similar 440 Gr lead gas checked bullet, I don't see a real issue for getting started.

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243winxb said:

Load 1 each at 35 grs , 36, 37, 38. with your 460 gr. Watch for pressure nearing 38 grs. The powder is going to burn just fine at 40,000 PSI.

Just what i would do.

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This

After reading the OP, I reviewed my 500 mag DB and figured 34gn to 37gn H110/296 would be a very safe/reliable load using the 460gn bullet.

Also, the 500 mag is actually a 60k rated cartridge and Accurate has many loads listed up to 57k.