Black powder grain size

A question has occurred to me concerning the use of black powder related to its use in pyrotechnics and firearms.

Why, as per specifications related to pyrotechnic black powder rocket construction and the loads for a primitive firearm, are there references to the specific grain size that should be utilized, i.e. : Fg, FFg, FFFg ?

The fundamental burn characteristics of these separate (uncompressed) powders is understood…

My confusion is derived from the fact that when one “rams” the powder, whether in a black powder rocket, or a primitive firearm, the original grain size is destroyed, resulting in a new, uniform, “grain” that is characterized by the compartment it has been packed into. So what makes the difference what size grain one starts out with?

Simplest explanation. The grain size doesn’t change when pressing or packing. The density of the grains in a cartridge case or weapon chamber changes. The grains aren’t crushed.

Confinement, grain size and density affect the rate of deflagration from grain to grain.

Explanations can be found at:

In the ‘Sharpe’ books, Bernard Cornwell has the riflemen grinding powder as fine as they could when they wanted more consistency.

Is this an accurate description?

I don’t have much experience with rockets, but I do shoot black powder weapons fairly regularly. I own a replica 1853 Enfield caplock rifle-musket (.58 cal), a replica 1756 long land pattern British musket which is a smooth bore flintlock (.75 cal), and a replica 1851 Colt Navy cap and ball revolver.

Despite what you see in Hollywood, you don’t ram the bejeezus out of it when you load a black powder firearm. You’re just pushing everything to the back end of the barrel so that there are no air gaps (an air gap in between the powder and the bullet basically turns your barrel into a pipe bomb). You don’t ram it anywhere near hard enough to crush the grains. Even if you do go all Hollywood style on it, you aren’t going to crush the grains much.

Finer grain powder burns faster, releasing more energy in a shorter amount of time. It also burns more completely, leaving less residue behind. But then it also takes more effort to manufacture the finer grain powder, so it’s a bit of a tradeoff.

Sorry for the delay in my reply, it took time for research per smithsb’s link, life’s interruptions, etc.

Differing black powder grain burn characteristics seem to be a matter of opinion to a great degree. US military research into this matter seems to be the most comprehensive, and their conclusions are less than definitive.

My inclusion of firearm loads and rocket “ramming” together was perhaps unrealistic related to my question… Pyro rocket construction involves much higher compression (3000-6000 lbs/in. squared) than is experienced in firearm loading.

That being said, I wonder: At what point (pressure) does commercial powder loose its individual grain characteristic in a muzzleloader or cap fired revolver.

I’ve made my own black powder for several years now, with varying grain hardness that is determined by my technique, and have noticed no substantial difference (chrony verified) in performance when loaded by weight… Hence the original question.

engineer_comp_geek : The manufacture of finer grain powder takes no more effort than the coarser grains. It’s just a matter classification with progressively finer screens to separate the same product.

The powder doesn’t lose it’s individual grain characteristics with density. Here’s another study that explores composition, density, grain size, porosity, etc…

You’re correct in that black powder remains more of an art than science. The best solution to date is:

Test samples from an individual production lot. If test does not match requirement, blend with another lot with opposite characteristics and retest.

When the accepted lot of BP is combined in a component such as a delay pellet, expelling charge, or time fuse; also test that lot for performance before acceptance and inclusion in next level component or end item.

From the text:
“This process (black powder mixture) has been standardized to some degree, in accordance with prevailing technology, but the operator adds water, adjusts temperature, selects grinding time, or changes compression pressure based on personal experience which is guided by individual judgment of color, odor, or general perception of the condition of black powder as it undergoes its various transformations. Such judgments are exchanged from experienced operator to a journeyman.”

Keep packing plenty of fine grains and eventually, you’ll see a difference. You’ll feel the difference too.
It goes back to the square-cube law. If you divide the dimensions of you grains by a factor of 2 and pack the same total weight, there is now 4 times more surface area*. Powder burns on its surface. That gives you more surface burning at the beginning of the burn and thus a greater pressure/heat spike near the beginning. It’s why you don’t use pistol powder in rifle rounds**.

If you gun can take the pressure spike, no problem. If not, you gun and face might be worse for it.

If you’ve been using larger grains with no change, perhaps your barrel is long enough that the larger grains have time to burn nearly completely.

*It’s also part of why espresso is stronger than regular coffee. The square-cube law is everywhere!

**If you do, please shoot a few hundred rounds while someone else films it and posts a Youtube link here.

According to your provided links, I beg to differ. Density does affect black powder burn profiles, and density does increase with the amount of compression.

The sources you refer to, tend to implicate that grain size burn characteristics tend to merge when higher compression ratios are achieved… My question is, at what density does the original grain size become irrelevant.

My guess is about 1.7 grams per centimeter squared.

Thank you for the links.

For the case of rockets, compression of the grain is likely an important issue. A rocket motor doesn’t burn in one hit, it burns on the surface and the burn rate is important for the flight profile. For a given energy in the grain you want to control the rate it burns to control the acceleration. Curiously the burn rate of a solid motor is proportional to the gas pressure in the motor as well as the grain composition. Geometry and composition will control the burn profile. But you are not looking for a motor to behave like a gun. Given the energies involved it can’t - it would just behave like a bomb (rapid self disassembly.)

Why would a few hundred rounds of my activity on YouTube interest you, other than boring, inconsequential activity?