Ganderite,
How does the chemistry change between different powders?
Single base (Nitrocellulose only) versus Double base (NC plus Nitroglycerine) is easy enough to understand.
But what makes one single base burn faster than another?
Shape? (stick, flake, ball, flattened ball, etc)
Size of grains? (Definitely so for black powder, with Fg FFg FFFg and FFFFg)
Surface coating like flame retardants etc?
Or is there something in the chemistry itself? So not all NC "dough" is created equal...?
And how similar or different is the Can. IMR 4895 to the Aussie H4895?
Thanks.
Single base powders vary in speed by deterrent coatings and kernel shape. DNT (not to be confused with its cousin "TNT") is probably the most common deterrent, but there are others, and sometimes more than one is used. The thickness of the coating controls the initial burn rate.
Think of a large kernel of powder like a log. It burns from the outside in. Initially, the large surface is burning, giving off a lot of gas. As the log gets smaller, the surface area is smaller, so less gas is generated.
Now consider the needs of the cartridge. Initially the powder is confined to the case, and the bullet is stuck in the case neck. The last thing you need is the full surface area of all the powder burning at full tilt. hence the deterrent, to slow it down.
Once the bullet starts moving, the volume behind the bullet gets larger, dropping pressure, so ideally you want the powder to start burning faster, to make up for the increased volume. Over the first inch or two, that happens. as the burn speeds up as the deterrent layer is burned off.
Then the bullet is rapidly moving down the barrel, dropping pressure, and the kernel surface is getting smaller, generating less and less gas. This is not good.
A well designed powder is progressive. That is, it burns faster and faster, not slower and slower. Deterrents help, and also the kernels have a hole tough them. Think of a fire log with a 1" hole drilled end to end. The kernel surface burns on the inside, too, and this increases the burning surface as it burns, helping to offset the outside surface getting smaller.
If you start with a vat of powder dough and extrude it in two sizes (larger diameter and length - as in 4895 and 4064) the larger chunk powder will burn slightly slower. This can be modified by changing the deterrent treatments.
Once upon a time I convinced a powder company that they should try short cutting some 4831 so that it would meter better. They did not know what a powder thrower was, so had no idea about metering characteristics. (They were chemists - not shooters.) They made up a test batch of shortcut 4831 and two other powders. (4895 and 4064).
The powders were tests in the lab (pressure gun) and the boss phoned me to report the tests were a total failure. The results of the shortcut powders were identical to the regular version.
I tried to explain that if the results were identical they should immediately switch over to the short cut version. They refused to see any advantage. They gave me the short cut 4895 (about 900 pounds) and scrapped the rest. I still have some of it.
One of the limitations of ball powder is that the surface area gets smaller as it burns, so it relies much more on deterrents to control the initial burn rate.
A double or triple base powder can have more energy, usually at the cost of higher flame temperatures, which speeds up throat erosion.
The chemist can use the composition of the dough, the size and shape of kernels, the size of the internal hole in extruded powders, and the thickness and composition of deterrents to control the end result.
I have seen powder that looked like fly sh!t and other kernels the size of ping pong balls.
