Interpolation of Powder Load Data?

[zardoz]

A question here for the internal ballistics experts.

In my Lyman manual, for say the 380 Auto cartridge, I find data for a 90 grain and a 120 grain cast boolits (Lyman moulds).

Now, I have a 105 grain Lee SWC mould, and have cast several of those.

The 105 grain weight falls exactly halfway between the 90 and 120 grain weights, being 15 grains on either side.

Looking further, for say a powder like Unique, I see that max load for the 90 grain is 3.7 grains (CUP=16K), and max load for the 120 grain is 2.8 grains(CUP=15.6 K).

Question is, can one safely assume a linear relationship in this case, and figure that a max load for the 105 grain could be 3.25 grains, which is in the middle of the two tested loads?

Interpolating the starting loads of 2.2 and 3.1 grains respectively, I get 2.6 grains starting load there for the 105 grain weight.

Is this relationship linear, curved, or mostly linear to a certain point, and then exponential increase in pressure?

What things can I safely assume given the listed data?

Thanks for your patience with my mathematical questions here.

[454PB]

I'm no expert, but there is more to consider than just boolit weight. Bearing surface and seating depth also must be factored in. Being the cautious type, I would use the data for the heavier boolit as a starting point.

I once had a 9mm kaboom, and since then I'm much more cautious with small capacity, high intensity pistols.

[Lead melter]

Maybe the best to go along with 454 on this one. Use the data from the heavier slug just to be on the safe side.

Maybe consulting another manual will give you info of another powder with the same boolit weight, but no doubt will not be of the same configuration. Once again, best to err on the side of safety.

[Rocky Raab]

The relationship is generally linear in instances like this. Not quite, but close enough to interpolate Start loads at least. Maximums might slide either way because of factors like bearing length and such already noted.

Book load data seldom or never EXACTLY matches field results anyway. Just the normal 3-5% allowable variation in powder lots will account for detectable change in velocity (and pressure if one can measure it). Other variables in bullet characteristics can account for significant variation, as can case volume, primer brisance and even weather.

[KYCaster]

Zardoz asks: "Question is, can one safely assume a linear relationship in this case, and figure that a max load for the 105 grain could be 3.25 grains, which is in the middle of the two tested loads?.....Is this relationship linear, curved, or mostly linear to a certain point, and then exponential increase in pressure?"


Answer to the first part is...Yes, if you're talking about Unique.

Answer to the second part is.....D.....All of the above.

GENERALLY if you're working in the middle of the suitable range of applications for most powders, your interpolation would be safe...but you have to be aware that not all powders are created equal.

Some, like H110/WW296 get kinda squirrely at the lower end of their range and they can be very sensitive to changes in boolit weight, neck tension and seating depth.

Others, like Blue Dot and Universal are prone to drastic pressure spikes as you approach the top end of their range, so small changes in overall length can mean the difference in a marginally safe load and one that's grossly over pressure.

Still others, don't like to be compressed, so a change in boolit style with the same weight (spitzer to wad cutter, eg.), that drasticly changes the seating depth could have a tremendous effect on pressure.

One of the great things about Unique, Herco, 2400, HP38 and some others is that within their suitable range of applications, their charge weight to energy ratio is pretty much linear from bottom to top, so with these you can interpolate data with some confidence.

Now, having said all that, I'll say that I believe the best policy is to use only data that is available from at least two different reliable sources. For example; Hornady publishes data for their bullets using powders from a variety of manufacturers and most of the powder companies publish data using Hornady bullets with their powders.

I realize that if you follow that advice you'll have a problem finding suitable loads for some of your favorite cast projectiles...that's why you're here asking about interpolating data. Just be aware that any load you see quoted on the 'net, may or may not be safe; especially if it is preceded by, "My log book is at home, but IIRC....." or, "My velocities were a lot lower than the book, so I...."

I hope that answers your questions.
Jerry

[StarMetal]

Just send an email to Ben Amonette at Ben.Amonette@ATK.COM
He's the engineer at Alliant Powder Co. Just ask him what you the question you asked here.

Joe

[Rocky Raab]

I think that reloaders are gradually waking up to the added fact that Alliant powders are NOT the same as Hercules powders of the same name.

Much frustration and confusion has arisen due to recent changes in load data, warnings and usage limits (the Blue Dot warnings as example). Reloaders say "I've used x grains of PinkDot for 30 years and now they tell me that it's over maximum? BS!"

But Hercules powders are gone. Alliant powders are made by different companies, in different plants, by different people, to a different recipe using different ingredients. They are DIFFERENT powders.

New powders mean new rules, new loads and new results. If your can of powder says Hercules, you can use data developed for Hercules powder. But if your can says Alliant, you should use data developed by Alliant. It's that simple.

[zardoz]

Thanks to all for the advice and information.

I want to be cautious in these things of course, having viewed a few of the KABOOM incidents online, and the results.

I take a close look at all my brass, even though the recoil seems mild compared to a factory loading, for pressure signs. I also weigh each and every powder charge in EVERYTHING so far. I do not trust auto powder measures, powder charge bars, etc. I also recalibrate my scales, and cross check using another scale, each session. Maybe paranoid, or overly cautious, but I need my eyesight, fingers, etc. for a long time.

In some of the once fired brass I buy for reloading, I have noted some flattened primers, and slight "guppy" case bulges, so it seems like there are some folks out there that push the limits. This has to be hard on autoloading pistol actions.

I just ordered a chronograph earlier this week, and eagerly await its arrival tomorrow via UPS. By cross referencing several loading charts, books, and just paying attention to all the common sense stuff, I hope to keep things as safe as I possibly can.

I've even reweighed my cartridges and shotshells after completing the loading process to triple check my work. This is nitroglycerin based stuff we're working with here, and it certainly has my respect.

Thanks again.

[Rocky Raab]

Let me tell you up front that a chronograph will cause you more head scratching than you've ever had. If you are the curious but careful type (as it seems you are), that little box will create more questions than answers - and it will deliver a LOT of answers.

Just be prepared!