The 1:8 barrel twist

[markm]

However, in simplistic terms, ball propellants are the mildest for throat erosion, then single based stick, and last double based stick.

Wait what? Ball is easiest on throat erosion??? I've read that the ball powders are "barrel killers". But in that context is was the harder bore fouling.. not so much throat erosion.

[lysander]

There is a reason 90% of all military small arms (30 mm and smaller) ammunition is loaded with ball propellant, it is the easiest on barrels. There is a ton of test data showing barrels that use ball propellant exclusively out last other propellant types.

There are two reasons for this, 1) ball propellant is heavily deterred to achieve progressive burn characteristics, so it's flame temperature is a good bit lower than other propellants, and 2) the calcium carbonate left over from processing is very good at reducing throat erosion, but it does make the propellant "sooty".

[markm]

I've surely read that ball powder is more robust than extruded... which makes sense for military ammo. Interesting info though.

[1168]

There is a reason 90% of all military small arms (30 mm and smaller) ammunition is loaded with ball propellant, it is the easiest on barrels. There is a ton of test data showing barrels that use ball propellant exclusively out last other propellant types.

There are two reasons for this, 1) ball propellant is heavily deterred to achieve progressive burn characteristics, so it's flame temperature is a good bit lower than other propellants, and 2) the calcium carbonate left over from processing is very good at reducing throat erosion, but it does make the propellant "sooty".

Why the difference between single-base and double-base extruded? Given the burn characteristics you’re describing with ball powder, I’d expect double-base to be closer to ball?

[vicious_cb]

Actually, there is an abundance of data on the subject, it is just that little of it is published in layman's terms, or easily accessed.

However, in simplistic terms, ball propellants are the mildest for throat erosion, then single based stick, and last double based stick.

Simplistic is good enough for me, Im not going to start taking apart my factory ammo to see whats inside other than the occasional bullet set back or bent necks but when it comes to double based stuff like vihtavuori I doubt thats going be used in high enough volume for it to matter. I was more concerned about whats in the cheap stuff like Wolf that gets shot in high volume since I have no clue what the characteristics are of the powder.

[markm]

I was more concerned about whats in the cheap stuff like Wolf that gets shot in high volume since I have no clue what the characteristics are of the powder they are using.

That Russian stuff is actually high quality ammo. Wolf is surely (but I could be wrong) extruded. I pulled some Herters steel cased apart once and subbed each component one at a time to see what made that Russian stuff tick.

Everything about it was good quality except the shitty bullets. I directly subbed 69gr match bullets over the existing 62 gr herter's charge, case, and primer, and that ammo shot well!

[vicious_cb]

That Russian stuff is actually high quality ammo. Wolf is surely (but I could be wrong) extruded. I pulled some Herters steel cased apart once and subbed each component one at a time to see what made that Russian stuff tick.

Everything about it was good quality except the shitty bullets. I directly subbed 69gr match bullets over the existing 62 gr herter's charge, case, and primer, and that ammo shot well!

I don't doubt it, also Im pretty sure thats all this stuff is: Russian case, primer, maybe powder topped with a Hornady bullet thats stupidly accurate for the price.

[lysander]

I've surely read that ball powder is more robust than extruded... which makes sense for military ammo. Interesting info though.

Small arms propellant needs to have progressive burn characteristics. As the bullet moves down the barrel, the volume increases, in order to keep the pressure increasing as this happens, you need to create gas faster than the volume increases, or at least as fast as the volume increases. There are three ways to do this, 1) by increasing the amount of propellant burning (increase the surface area on fire), 2) have the area that is on fire, burn faster, or 3) a combination of (1) and (2). Number (1) can be done by the geometry of the propellant grain. Number (2) can be done by slowing down the initial speed the material burns, so that the later rate is relatively faster.

To achieve (1):
Look at a cylinder, as the surface burns away, the radius of the grain decreases, which means the surface area decreases, if you put a single hole in the center of the cylinder (single perforation grain), as the grain burns, the outside area decreases, but the inside hole gets bigger so the total area burning remains mostly constant. If you put several holes in the grain (multi-perforation grain) each of the holes will grow as the outside shrinks, and total area increased.

To achieve (2):
If you coat the surface of the grain with a "flame retardant", or "deterrent", the flame will burn slower until it burns through the deterred layer.

For (3):
By using both grain geometry and a deterrent you can get single-perf grains to burn progressively, even though the area is remaining constant.

If you look and a sphere, the area decreases rapidly as the diameter decreases during burning, so the surface needs to be heavily deterred in order to get progressive burning. And, as a result of this thick coat of "flame retardant" to reduce the burning rate, you get a reduction in the heat liberated in relation to time, so the flame temperature is lower, even though ball propellants are all double based.

Also, spheres pack better with less dead air space between the grains so you can physically get more flammable material in the same volume. This makes ball propellants much more "energy dense", so you can get higher velocities from the the same volume, assuming everything else is equal.

Single base propellants are 99% nitrocellulose, double base propellants have about 10% nitroglycerine, giving them a higher energy content, but at the cost of a higher flame temperature.

For example the deterred flame temperatures for selected propellants:

WC 846 - 2244° K
IMR 4227 - 2550° K
IMR 4350 - 2622° K
IMR 4895 - 2287° K
IMR 8138M - 2406° K
EXP 1591A - 2912° K

All of the IMRs are single based, and the EXP is a double based extruded propellant.

Old Army films are fun and informative:

[georgeib]

Another great post. Thank you, Lysander.

[markm]

Interesting... and possibly the explanation of how a mil pull down 5.56 has SO MUCH powder inside. If I'm tracking the powder has a certain retardant to allow for more propellant without an early pressure spike that could burst the case or worse.