formula for powder charge

[George]

There has to be some real physical relationship between caliber and twist for the formula to make sense. Otherwise it's just one of those math game tricks where you are told to go through a series of nonsensical maneuvers which always results in you age. I don't know of any such relationship, but if there is a real one, I'd like to know it.

A mathematical formula is just a shorthand way of telling a story, but the story has to make sense, not just generate numbers which seem significant but aren't.

Spence

 

[mooman76]

I myself don't really like the short cuts. Half the fun is working up loads and if you don't use a wide range, you really don't know what your gun can do and it's limits.

 

[bender20]

I'm with Spence on this one. Just because your formula seems to "work" it doesn't mean anything. You need a reason for why you do what you do. Your formula is stating that there is a relationship between the caliber of the barrel and the rate of twist. How did you come with that fact that you multiply versus divide or how did you know the relationship was linear? It also only provides you with a "starting point" which is only moderately useful. Most people will tell you to start at 1.5 times the caliber that is also an equation but doesn't explain anything it is simply a rule of thumb. Spence said it best when he said an equation is just a shorthand way to right a story. So the question becomes what is the story your equation tells?

 

[Tom Compton]

It's my understanding that a PRB of any caliber needs a certain spin for accuracy. I plugged in the info for .40, .50 and .54 and the twists in my barrels. I only use FFg. Great estimates for starting loads. I'd try 10 gr less and work up from there. Reworking an ignored .36 and upgrading it to flintlock. When finished I'll determine the twist and plug in to this.
Thx
TC

 

[mr.flintlock]

Well, I am not a scientist but I ran across this formula several years ago but don't remember where. I do know that an elongated bullet needs more revolutions per second to stabilize than a round ball and 3f powder burns faster than 2f and creates more pressure. You can chronograph loads with same bullet weight and same powder loads with 2f and 3f powder and 3f loads will give higher velocities and that must be the reasoning for the formula using 3f and round balls only. I have rifles ranging from .32 to .58 and this is the formula that works for all of them. The last deer that I killed this past hunting season was killed with a .54 caliber rifle with 1/70 twist and I used a70gr charge of 3f. I arrived at this charge by using this formula. My rifle is very accurate with this load and kills cleanly.

 

[George]

Sometime back we had a discussion on this same topic. The gist, a charge of 11.5 grains per cubic inches of the barrel volume. There seems to be no need to get fancy, just calculate volume based an bore diameter alone and barrel length.

What you are referencing is the Davenport formula, which is a perfect example of what I mean when I say a formula has to tell a story which makes sense. That one doesn't. It says any barrel can 'efficiently' burn 11.5 grains of powder per cubic inch of bore volume. It doesn't specify what granulation of powder, what kind of projectile, or any other qualifiers. It doesn't tell you where the information that a barrel can 'efficiently' burn 11.5 grains of powder came from, how it was determined, or what burning powder efficiently even means.

If you check out the results you get with it you see that it can get you in trouble.

The formula is πr2h, PI x radius squared x height, that result x 11.5. Using my 20 gauge smoothbore with a 46-inch barrel as an example, that's 3.14 x .3075 x .3075 x 46 which gives 13.66 cubic inches of bore. If I shot 11.5 grain of powder for each cubic inch I'd load 157+ grains.

I don't want to shoot 157 grains of any granulation.

I don't think this is the sort of information which should be suggested for someone looking for a way to figure a safe starting load.

The majority of the formulas floating around the BP shooting hobby are garbage. They don't make any sense, but they seem all sciency, so are accepted and passed along as true information about how our guns work. They are not.

Spence

 

[tenngun]

Remember though that this is for fffg powder and round ball only.

And .54 caliber and 1/66 twist also....

When you apply it to other calibers and twists, the numbers get a little fuzzy.

And it doesn't work at all with smoothbores.....unless I supplement 66 as a constant...

The old 1 grain per cal is good for a moment. 35(36 grains will work but might be heavier then you need in a .36, 45 in a .45 can work good but then so can 1&1/2. 50 might be too light in a .50, but not a load killer, and within range will make deer in to venison. 100 is not unreasonable. from .45 to .62 my guns have all done well with between 60 and 90 grains.

 

[coloradoclyde]

Considering I've shot from 5 to 150 grains of powder out of the same gun, saying something "will work" is a moot point.

Some 50 cal guns have a 1/28 twist....According to the formula the optimum powder charge would be 28 grains...

I say :bull:

 

[mr.flintlock]

Actually, a 50 caliber rifle with a 1/28 twist is not a barrel made to shoot round balls however with a 30gr load of 3f it will shoot pretty accurately out to 25-30 yards. Of course, this would only be a target load. Also, I never said that this formula would give the optimum load. I said it would give a good starting load and that it would need to be fine tuned.

 

[coloradoclyde]

The "formula" only has the appearance of being mathematical....
Which is sufficient for convincing newbies...

 

[tenngun]

I never had an ml with such a fast twist. All my guns have been slow or non-existent. So maybe some formula could work with it. However my experince with guns of .50 + keep dropping in the 60-90 grain range. What works for smaller guns breaks down on bigger. I found 1/3 ball wt a good starting point too. About 20 in a .36, maybe 25 on up to 30 was good. 40 in a .45 worked well though more might be needed, 60+ in a 50, 80 in a .54 100 is ok for a .62 and a250 was in the service load for s Bess. :idunno:

 

[coloradoclyde]

Even if you give the formula a hypothetical rate of twist like 1/500 rather than zero, the formula doesn't work.....

It only gets you close, using 66 or a number close to 66...
That's not a formula! only the illusion of a formula.

 

[Zonie]

I believe any time you have a written mathametical statement like, X + Y = Z or A/B = C it can accuretly be called a "formula".

Now, whether the formula is worth anything is another thing. :hmm:

 

[coloradoclyde]

Now, whether the formula is worth anything is another thing. :hmm:

And right there is the point.....It is worthless because it is neither accurate, predictive, or consistent.

It only works within a narrow range of numbers and the results are an opinion backed up with words like

"Close"

Would you have used such a formula when you worked in the aerospace industry? :hmm:

 

[Zonie]

I've used hundreds of formulas in the aerospace industry.
Most were good. A few were not.

The point of my comment was, when you said, " That's not a formula! only the illusion of a formula." your statement was not true.

Had you said something like, "The answers your formula gives are just an illusion of the truth", I would have been in total agreement. :v

 

[bender20]

Point taken zonie you are correct. I amend my statement to reflect your point. It is a formula but it doesn't mean anything.

 

[George]

Is it a formula if it tells a lie?

Spence

 

[Zonie]

Sure it is.

A formula is just a guide or rule to be used to calculate something. It may be totally wrong for what you want to calculate or it may be wrong for doing anything.

Think of "formula" like the word "sentence".

We write using sentences. We calculate using formulas.

A sentence can be totally wrong in what it is saying and a formula can be totally wrong in what it is calculating.

Right or wrong, they are still sentences or formulas.

 

[coloradoclyde]

Had you said something like, "The answers your formula gives are just an illusion of the truth", I would have been in total agreement. :v

Thank you for correcting and clarifying the point I was trying to make.... :thumbsup:

 

[Toomuch51]

The best formula I've been able to find, and the one I recommend to all my customers, is: start with 1 gr per caliber (50 cal. = 50 gr., 54 cal. = 55 gr. ect.) shoot for group then adjust 5 gr each direction shooting for group each time, continue with this process until you get your tightest grouping this will be your "sweet" load, this will not necessarly be the same for all rifles even those of the same model. Once you have reached this point then, and only them, you are ready to start adjusting your sights. :thumbsup: (rate of twist is not a variable) (size of ball and patch thickness though are also very important)