Powder charge vs barrel length

[ncmtmike]

This question is for the ones that know how powder burns at what rate. If,,you where using 70 grs of fffg in a 21" barrel vs a 32" barrel would it all ignite? Or would you have blow-off of excess powder?
Just been setting here studying on canoe guns vs standard length rifles.

Thanks :wink:

 

[zimmerstutzen]

It isn't just barrel length, but also caliber. A 54 or 58 can burn much more powder in a given barrel length than a 40 or 45 caliber. The space (inches of barrel)taken up by the charge is less in the big bore too

 

[ncmtmike]

These would be .50's I speak of. :wink:

 

[Col. Batguano]

Flint?

Flintlocks yield about 5-10% slower velocities than percussion with the same powder charge due to the fact that more of the pressure gets relieved through the touch hole than in a percussion gun venting out the nipple. That also means you can load more powder in them before you reach the maximum for barrel length place.

The short answer to your question is "probably", but there will still also probably be some embers that are lit when they exit the muzzle. Best way to find out is to shoot it and chronograph it. Night time is always fun. Once you quit getting increased velocities you know you've hit the practical limit, beyond which you won't get much more of a performance boost for an increased charge, and, velocities will have big swings. A total WAG on that spot (I'm assuming a tight PRB on the 21" tube) would be in the 110 gr 3F vicinity. But it's a WAG, not based upon anything measured.

 

[coloradoclyde]

I saw a chart once but can’t remember where (old Lyman manual maybe). Zonie probably knows.
But I’m not sure any BP firearm really burns all the powder when you look at high speed photos. It just becomes a matter of how much powder you’re wasting.

 

[Billnpatti]

I think you may be referring to the Davenport formula. This is a controversial formula because it does not take into consideration several important factors such as granulation of the powder and brand of the powder (different brands have different burning rates). However, it was developed by a man by the name of Davenport who shot in many matches at Friendship and was considered an outstanding marksman. When using his formula, what you are getting is an approximation of the maximum efficient load. That means the amount of powder that will be expected to completely burn within the bore of the rifle without any unburned powder being blown out of the muzzle. Remember, it is not a spot on number, but just a decent approximation. Here, then, is the formula: Pi times the radius of the bore squared times the length of the bore times 11.5 equals the grains of powder that constiture the maximumn effective load.

An example: Let's assume that you have a .50 caliber rifle with a barrel length of 41 inches. The radius is 1/2 the bore diameter so in this case, it is 0.25 inches. You square 0.25 and you get 0.0625. You multiply 0.0625 by 41 which gives you 2.5625. You multiply 2.5625 by Pi (3.1416) and you get 8.0504. You now multiply this number by 11.5 and you get 92.6 grains. This means that a .50 caliber barrel that is 41 inches long will have a maximum efficient load of 92.6 grains according to the Davenport formula.

It is important to remember that the Davenport formula does not take into account some important factors so it is just a decent approximation of how much powder a given barrel will burn efficiently. That is, how much powder it will burn before it starts blowing unburned powder out of the muzzle.

 

[Dave Rosenthal]

Short answer: YES, it will mostly all ignite.

Your research will be enlightened if you come across any Paul Vallandigham writings about the famous "Davenport Formula". It is a mathmatical formula which takes into account the barrel's diameter (caliber) when tracking the point of diminishing returns as far as how much powder will completely burn in a given barrel length. Additional powder will burn, but leave behind more fouling and some powder may be re-lightable after recycling.

My 28" barrel of .50 cal. will efficiently burn 72.5 grains of FFFg powder, so that's what I feed it since THAT is a "sweet spot" accuracy-wise. If I add an additional 10 grains of powder, I won't get the exact same increase in ball speed (for those 10 grains) as when I went from 62.5 to 72.5, and THIS is how one determines the "point of diminishing returns". If I put 100 grains of powder in the tube, the PRB will go a lot faster than if I used 80 grains or 85 grains, it just won't be as much speed PER grain, thus the "diminishing" part.

Davenport Formula goes back to rifle training at some Army Fort that I can't remember right now. The formula WORKS for PRB's out of rifles!

Dave

 

[Dave Rosenthal]

I couldn't locate Paul's writings in the archives. While I was lookin' for them you posted, lol!

He's been gone 2 years now........

 

[Bob J]

I think you may be referring to the Davenport formula. This is a controversial formula because it does not take into consideration several important factors such as granulation of the powder and brand of the powder (different brands have different burning rates). However, it was developed by a man by the name of Davenport who shot in many matches at Friendship and was considered an outstanding marksman. When using his formula, what you are getting is an approximation of the maximum efficient load. That means the amount of powder that will be expected to completely burn within the bore of the rifle without any unburned powder being blown out of the muzzle. Remember, it is not a spot on number, but just a decent approximation. Here, then, is the formula: Pi times the radius of the bore squared times the length of the bore times 11.5 equals the grains of powder that constiture the maximumn effective load.

An example: Let's assume that you have a .50 caliber rifle with a barrel length of 41 inches. The radius is 1/2 the bore diameter so in this case, it is 0.25 inches. You square 0.25 and you get 0.0625. You multiply 0.0625 by 41 which gives you 2.5625. You multiply 2.5625 by Pi (3.1416) and you get 8.0504. You now multiply this number by 11.5 and you get 92.6 grains. This means that a .50 caliber barrel that is 41 inches long will have a maximum efficient load of 92.6 grains according to the Davenport formula.

I just tried this out for a 31 inch .62 barrel and came out with 108 grains. Interestingly, after much shooting from 80 to 120 grains, I found the most accurate 2f hunting load to be with 110 grains (and found that 90 grains 3f approximated same group and POI). That being said, when dropped all the way down to 50 grains, the rifle again grouped well - but this is a target/practice load.

Tried the Davenport formula for a 42 inch .36 barrel and it came out to 49 grains. This rifle shoots very accurately with 40 grains 3f which would be darn near equivalent to 49 grains 2f. It will not shoot a good group with 25 or 30 grains, I've tried. So there seems to be some merit to the Davenport Formula, at least in my humble estimation.

 

[zimmerstutzen]

If you check the charge caliber and barrel length tables in the first edition of the Lyman Handbook, you will see that for most usual calibers and barrel lengths there is a point at which the same increase in powder charge does not result in the same increase in velocity. In some fields this would be called the point of diminishing returns. it is similar to the Davenport formula

 

[coloradoclyde]

That's the one I was thinking of

 

[Dphar]

One must remember that there is considerable ejecta from a charge of BP and obscuring smoke that would make seeing much difficult so HS cameras might not do much. Most of its is incandescent either as gas or fouling. Having once shot up to 65 grains of FFF Goex from a 32" barreled 32 caliber over white sheets and finding no unburnt powder I decided that a RB rifle will burn a lot more powder than people might think. This shoot over sheets is based on (so far as I can tell) a fairy tale told to Ned Roberts by Brockway about slug guns. He was using coarse powder usually C&H #6.

 

[zimmerstutzen]

as a related matter, i have found that the more the point of diminishing returns is passed with powder, the more fouling there tends to be.

In addition to the other variables that can affect whether powder is consumed mostly within the barrel or outside the barrel as muzzle flash, is the place and type of ignition. For instance, in an underhammer, the spark is driven into the main powder charge, almost like a diesel injector. If the nipple is slightly ahead of the breech, the powder charge is burning from the middle toward the front and toward the breech at the same time. instead of burning from the breech toward the rear. Now all this occurs in milliseconds anyway. Whether it shaves a few milliseconds off the ignition and permits more complete combustion within the bore is something for which maybe Pletch has an answer.

In a short barrel, the powder charge takes up space. the difference between 50 grains and 100 grains is that the ball is moved twice as far from the breech and that much less bore to traverse before muzzle exit.

 

[Billnpatti]

This may be the posting by Paul that you were looking for. Paul was a very knowlegable man and one from whom we could learn a lot. I miss him, he was a good friend.

If the formula that people are referring to here is he one that Charles Davenport promolgated back in the late 40s, THAT FORMULA has NOTHING to do with " Maximum Powder BURN"-- whatever that is.

The Davenport formula- 11.5 grains of Black powder per cubic inch of bore-- has to do with finding quickly your MAXIMUM EFFICIENT POWDER CHARGE for a GIVEN CALIBER, AND LENGTH OF BARREL. The formula does NOT attempt to determine the maximum amount of powder that will "burn" inside a given barrel length and caliber.

"Efficiency" is defined, for purpose of the formula, as the continued similar, or greater, increase in velocity for a given increase in increments of powder used. The "Breaking point" is the " Maximum efficient Powder Charge for a given caliber and barrel. Flintlock rifles can use that powder charge, and shooters can then test loads 10% below to 10% above that recommended powder charge, shooting RBs, to find their most accurate load. Percussion shooters need to reduce that recommended charge by 10%, and then begin testing powder charges, 10 % below that number, to 10% above to find their most accurate load for that gun.

The difference between how you use the formula with flintlock, and with percussion rifles, is that flintlocks LOSE a certain percentage of velocity due to the presence of the Vent at the rear of the chamber. That loss can range from 5-10% of the velocity a similar load of BP fired from a similar caliber and barrel length in a percussion system, depending on the size of the TH( vent).

The Formula was intended to help TARGET SHOOTERS- not hunters--- find their most accurate load for their guns, while keeping recoil to manageable levels. TARGET SHOOTING MATCHES tend to be ENDURANCE CONTESTS, where recoil becomes a factor in deciding the winners and "also-ran's". Hunters rarely fire more than a couple of shots at game during a season, unless hunting small game, where bag limits are generous, and heavy powder charges simply are not needed.

The formula works for what it is- and NOTHING MORE. It has been misrepresented, and misused, HERE, on this forum, by people who have other motives in mind.
I AM UNAWARE OF ANY FORMULA that can give a clue as to the Maximum powder charge that will burn in a barrel of a given caliber and barrel length. When you consider the different burn rates of different kinds of Black Powder, based on changes in the percentage of chemicals used, and the size of the granules produced by different manufacturers, Even attempting to create such a formula would be "SILLY"! Throw in the substitute powders and you now have an exercise in Frustration and Futility.

Phil Quaglino, who is my source of information about Charles Davenport, and the "formula", uses the formula not only in his rifles, but also in his revolvers and pistols. Phil holds or held several National Championship Records in both Rifle and handguns.

For the typical .36 caliber revolver( ie. Colt 1851 model replicas), he recommends using 13 grains of FFFg Goex powder. For the typical( ie., Ruger OAA) .44 caliber revolver, he recommends using 18 grains of FFFg powder FOR TARGET SHOOTING. He uses those same powder charges in his revolvers when hunting rabbits or other small game with BP revolvers, taking head shots, at short ranges, to preserve as much meat as possible. Those powder charge recommendations are based on the "formula".

 

[Dave Rosenthal]

THANKS very much! Paul had a way about him like no other before or since. He could take a very technical subject & break it down into easy to understand common english, like he did here!

And FWIW every single person that I've taught this to has improved their scores! Some folks just don't like to admit that SCIENCE has a lot to do with accuracy. Paul was one of several of my mentors. I never met him in-person, but we did chat on the phone & PM each other quite often. I miss him!

Dave