Formula for figuring out the powder charge

[MXD]

I read on another forum that there is a formula for figuring out the powder charge between 2 bullets using the same powder, design, shape and depth but different weights.

For example, if I had a 115g rn and I knew my confirmed recipe called for 5g of powder, if I then went to a 147g but did not have a recipe for it, I could use the following equation for a starting point (minus a few percent of course):

Square root of (larger bullet weight divided by smaller bullet weight)
For lighter bullets, you would multiply that number by the original charge and for heavier bullets, you would divide it

So if I was working with 115g RN using 5g of powder and I wanted to find the right powder charge for the same bullet at the same depth with a weight of 147g the equation would be:

147/115=1.278. The sq. root of 1.278=1.13
the proper charge for the 147g bullet would be 5/1.13=4.42g

or if the 147 was 5g and I was looking for the 115:
147/115=1.278. The sq. root of 1.278=1.13
the proper charge for the 115g bullet would be 5x1.13=5.65g

Has anyone used this? Is it terrible advice or is it a safe equation?

 

[rattletrap1970]

Unless I am mistaken, you cannot derive any data from a single data point. You would need at least one, preferably more information to guesstimate your charge. The pressure and rate of increase may not be linear, so more than two data points would be better. If you had a chronograph you could start at the lighter load and incrementally increase your powder and record your velocities. There is plenty of data online that will give you an idea of what velocity you should be achieving with a given weight bullet. Adjust your load to approximate that. That would be best.

 

[EddieCoyle]

I'm going with "terrible advice" on this one.

The pressure curve varies greatly from powder-to-powder. So much so that there's not going to be any formula that'll work for all powders.

 

[MXD]

I'm going with "terrible advice" on this one.

The pressure curve varies greatly from powder-to-powder. So much so that there's not going to be any formula that'll work for all powders.

The formula is for all things being equal except for bullet weight.

I just edited my first post to clear it up.

 

[Supermoto]

how is a 147 and 115 going to have the same depth?

 

[MXD]

how is a 147 and 115 going to have the same depth?

I don't know. Maybe he meant relative depth since the 147 will use less powder the "free space" between the powder and bullet is comparable?

 

[rattletrap1970]

I generally seat to the maximum safe OAL. The heavier bullets will of course generate more pressure as they are longer, thereby shortening the gap between the powder and the bullet base and having more inertia because they are heavier. You get more pressure and quicker buildup of pressure because of this. For the $199 it costs for a good chronograph it is an invaluable tool for those who like to experiment.

 

[EddieCoyle]

The formula is for all things being equal except for bullet weight.

I just edited my first post to clear it up.

Even still.

There are powders where the pressure curve skyrockets upward at a certain tipping point. There are other powders where the pressure increases much more gradually.

If you want to test it, go to Hodgdon's site and try your formula out on Trailboss and H110. I'd guess that on one or both of those powders, your formula will be way off when you compare it to the actual data shown on the site.

 

[MXD]

Thanks for the quick replies. I'm glad I asked. I feel like we just did a short "Mythbusters" episode LOL

 

[GTOShootr]

It is just silly. What were the responses to that post on the other forum?

You need to chronograph stuff in your guns and observe the results on the chrono, on the brass, in the gun, and on the target. You can't use a formula or even reloading books and acquire accurate information regarding what you are loading and how it is coming out of your gun under varying conditions. You also need to get some experience to understand what you are actually talking about in terms of "proper load" or "comparable load". Do you mean similar velocity, do you mean similar pressure, do you mean the same power factor, do you simply mean a similar feel?

 

[highscore]

Scares the hell out of me and I love to experiment.

 

[Lucas McCain]

Don't let this happen to you. No it isn't mine but I was at the range when it went off, not pretty.

 

[DukeInFlorida]

Don't guess or try to do any space age math when reloading. If you come up with a bullet that you can't find in a reloading data book, then call the powder manufacturer and have them tell you EXACTLY how much to use.
They have an obligation to give you accurate advice. (Including overall length for the bullet/powder)

 

[saltydude]

I'm going with "terrible advice" on this one.

The pressure curve varies greatly from powder-to-powder. So much so that there's not going to be any formula that'll work for all powders.

+1

There's too many variables too use a simple formula. The pressure curve & burn rate relationship are not always relative.
I know I'm one to talk but---
Sounds like a formula for destruction.
As others have said, Compile as much reliable data as possible, get a chronograph & experiment from there.

 

[highscore]

View attachment 10640

Don't let this happen to you. No it isn't mine but I was at the range when it went off, not pretty.

Looks like case head seperation caused by reloading too many times or headspace problem.

 

[RKG]

I join Brother Coyle in rejecting this approach. It is ultimately based on the notion that there is a reliable linearity to the charge/pressure relationship, and that is not always the case.

 

[Gammon]

I generally seat to the maximum safe OAL. The heavier bullets will of course generate more pressure as they are longer, thereby shortening the gap between the powder and the bullet base and having more inertia because they are heavier. You get more pressure and quicker buildup of pressure because of this. For the $199 it costs for a good chronograph it is an invaluable tool for those who like to experiment.

$199? A Competition Electronics Pro Chrono can be had for less than $100. They don't have all the "bells and whistles" but are quite serviceable. I have been using and older version of this chrono for about 25 years. I love the new models with all the options, but the old one just works too well to replace.

Chronos aren't just invaluable tools for reloaders who like to "experiment"; they are invaluable tools for all reloaders. Reloading data is very general in its application and your results with your guns may vary quite a bit from the published data. Without a chrono you really have no idea what you are doing.