Why does BP have to be compressed?

[Forestarius]

I have a bookshelf full of blackpowder books that say when loading a revolver the ball must be seated to the powder, compress the powder, add filler, etc., and leave no airpspace or "you'll be sorry!" But they never say why.

It obviously does not cause a problem to have extra air space in smokeless powder cartridges, so what is it about blackpowder that makes it bad? I am surprised in all these books not one author bothered to explain the why behind the warning. Is it just an old wive's tale or is there science behind it?

 

[Howdy Doody]

BP shoots cleaner when under a bit of compression and compressing makes for more even loads than having an air space. Most folks use the 1/8" compression by the bullet as a gauge. For precision shooting such as BPCR most use a compression die, drop tube and other tricks, like weighing vs measuring by volume.

 

[Old Dragoon]

Black Powder is an explosive and you do not want an airgap or it could rupture the chamber. if it is compressed (no air gap) it will not.

 

[gmatov]

BP is a deflagrating powder, not an explosive, except for ATF and shipping purposes. It burns relativily quickly, at 14 cm3, per second. Hardly an explosive.

A small airspace will not cause a blowup. A half lodged ball in a rifle barrel will cause a volume of air to be compressed, with an abrupt obstruction that distance up the barrell. THEN, you could have a problem with shock waves You quickly compress the air before the ball, and pressure can rise to the point it will bulge, or burst, the barrell.

A ball not seated deeply enough into the chamber of your Rem, with the counterbore in the chambers, a "parlor pistol" load, will not blow up your gun.

We get so much "Gospel" from these sites, it's a wonder ANYBODY shoots these bombs Just waiting to blow your limbs off.

Cheers,

George

 

[RyanM]

A small airspace will not cause a blowup. A half lodged ball in a rifle barrel will cause a volume of air to be compressed, with an abrupt obstruction that distance up the barrell. THEN, you could have a problem with shock waves You quickly compress the air before the ball, and pressure can rise to the point it will bulge, or burst, the barrell.

No, that's the pressure of the explosion alone doing that, pretty much. Air is a gas. Burning black powder is mostly gas. Doesn't really matter what it's made of in this case; gas can bulge a barrel if the pressure is way too high. The fact that there may be some air in there makes absolutely no difference.

The air gap thing is indeed a myth, though. Black powder (smokeless too) has an oxygen source mixed in (the saltpeter). A tiny bit of air really won't do crap. Sure there have been flour mill explosions due to suspended flour, but that's because flour doesn't contain an oxygen source, so it can only blow up if it's suspended in the air.

In fact, air spaces should be less of a safety concern with black powder than smokeless, because black will burn at the same rate regardless of the volume it's burned in. Smokeless burns faster in a small volume and slower in a large volume, so too big of an airspace can decrease the pressure so much that you get a stuck bullet. Then if you try firing another, BOOOOM!

 

[Ferret]

Aha... something that my physics background can answer.

Blackpowder is not a true explosive, no, but, neither are most smokeless. They are compounds that burn very quickly.

Air space in BP Cylinder / smokepole. (between powder and ball)
cap ignites
powder ignites
air (only part being friendly to fire.. ie Oxygen, hydrogen, etc)... some gets consumed by the burn, this will raise pressure a little. The rest of the air will compress. The air compression isnt gonna do squat if the airgap is small.

The problem occurs if you have the ball seated way up front and a relatively small amount of powder. This means that there is a large airspace, this means more oxygen etc, and more non volitile gasses to compress.
IF the ball doesnt give way first and exit the chamber, pressures CAN get high enough to rupture the cylinder.

Air space in cartridge
Primer ignites
Powder ignites
bullet is crimped (if it is crimped) into the case with FAR less force than the burning powder takes to unseat it. You can unseat a bullet with a primer sometimes.
If the bullet doesnt exit the case, the case walls will rupture and vent pressure before any serious harm to the chamber.

Air space in barrel - ie... stuck ball or bullet

THIS IS A PROBLEM!
cap ignites
powder ignites
ball (or bullet) goes sailing down the barrel.
If the barrel has a blockage, be it a stuck ball, or even hardened mud, there will be problems.
The air in the barrel between the station object and the moving bullet will get compressed. This pressure most times will not exceed the force that it will take to move a blockage. There will still be pressure build up behind the bullet that you have just fired. Some of this will vent past the case and back towards the shooter, or will pop the cap off the BP revolver or smokepole. I have seen muskets be recocked when this happens.
The pressure between the two bullets will have nowhere to relieve itself.
Barrel will bulge until pressure drops below the weakness point of the metal.

In OLD firearms, the steel was not as good as we have now and it took less pressure to cause problems.


NOTE - I am NOT stating that you should go out and try any of this.

 

[Low Key]

A little while back, the mythbusters tried to rupture the barrel of a modern shotgun using various obstructions such as fake human fingers, mud, and even welded a metal plug in the end of the barrel. They did get a bulge from the gun, but no ruptures of the barrel. I post this only to illustrate that modern firearms, replicas of bp guns included, are a bit tougher than we think they are. BUT, do not ever attempt to shoot a weapon with an obstructed barrel and as per the manufacturer instructions do not leave airspace when loading a BP load in either a pistol or rifle, follow all safety rules and try to err on the side of caution. It's your life and you only have one go at it, and I'd prefer to live mine with ALL of my appendages still attached.

 

[Beartracker]

For those of you who don't believe BP is an Explosive you are just a little off. Just because the explosion happens due to expanding gases does not make it a non explosive and in fact thats what causes most explosions.

EXPLOSIVES:
any substance or device that can be made to produce a volume of rapidly expanding gas in an extremely brief period. There are three fundamental types: mechanical, nuclear, and chemical. A mechanical explosive is one that depends on a physical reaction, such as overloading a container with compressed air. Such a device has some application in mining, where the release of gas from chemical explosives may be undesirable, but otherwise is very little used. A nuclear explosive is one in which a sustained nuclear reaction can be made to take place with almost instant rapidity, releasing large amounts of energy. Experimentation has been carried on with nuclear explosives for possible petroleum extraction purposes. This article is concerned with chemical explosives, which account for virtually all explosive applications in engineering.

 

[Third_Rail]

LOW explosive, but yes it's an explosive.

 

[unspellable]

air, bangs, and what not

First, the burning rate of any powder has nothing to do with the volume it's confined in per se. It's the pressure and temperature that influence the burning rate. You are of course, more likely to get more pressure in a smalleer space.

The buring rate of black powder certainly does vary with pressure. Simply try burning a pile of it in the open vs in your favorite shootin' iron for an obvious illustration.

As for air spaces and obstructions, the air has little to do with the burning rate since the powder carries its own oxidizer. What keeps getting overlooked here is that you do not have a static situation. If you plugged the barrel and filled it with compressed air through the nipple you would have a static situation with the same pressure every where in the barrel.

When you set off a charge of powder things happen very fast. It takes time for the gas to move and you do not have the same pressure every where in the barrel. Not only that, but moving gas has momentum. It's moving fast and it has a lot of momentum. If you have an obstruction in a shot gun barrel the shot load will slow down when it hits it and the gas will pile up behind it. That's why such cases are famous for producuing a bulge behind the obstruction rather than bulging the barrel else where.