Please explain BP burn rate?

[rajb123]

OK, so I read that BP burns faster than smokeless powder. But smokeless powder creates much higher pressures.

Isn't that counter-intuitive?

Also, barrel length and bullet velocities seem to be impacted more in BP guns than smokless. Is that correct?

Thx...

 

[samcolt1860]

It's all about CHEMISTRY!

 

[rcmodel]

In real simple terms that won't hold up in a chemistry lab?

Black power doesn't really have a burn rate when compared to smokeless powder.
It is a low grade explosive that gives up its energy pretty much all at once.
But the explosion is slow enough that it takes a long barrel for it all to explode as the flame front works its way through the charge.

Smokeless powder is a flammable, and is able to maintain increasing pressure over a longer time frame as it burns. Various burn rate powders can be tailored to the cartridge to give the longest pressure curve, and the highest velocity.

So, it works to push the projectile over a longer period in the bore then the "kick in the shorts" black powder explosion.

rc

 

[freedom475]

Black Powder is quite a bit slower than smokeless powder in a chamber.

With smokeless, pressure happens in the case before the bullet even leaves the crimp..

 

[BCRider]

Even directly considered as you posted it's not at all counter intuitive.

Black powder may or may not burn faster than some of the smokeless powders. But burn rate has very little to do with pressure. Instead the pressure comes from the amount of gasses created when the burn occurs. And simply put it takes a lot more BP to get the amount of gas needed to push a bullet at a given velocity.

For example a .45 Colt requires up around 40 grains of BP per load. But if you use something like Tightgroup it only takes around 5 to 6 grains to generate the same pressure to push the bullet to the same velocity. That's a fully filled case of black powder compared to a little "dusting" in the bottom for the Tightgroup.

Also because we fill them to capacity or even until the BP is slightly compressed the powder tends to burn from the back to the front in a wave through the slug of powder. This can easily be seen by the amount of still burning embers that are blown out the muzzle.

It's due to this compression that the powder is controlled. A serious safety factor for BP loads is that the powder MUST be lightly to moderately compressed. You do NOT want an airspace in a BP load. If you do too much of the powder is exposed to the flame front during the burn and then the pressure do indeed peak. Disastrously so if you are shooting something with a lot of powder where the bullet can't get out of the way quickly enough.

Taken another way.... I load 3.6 gns of Bullseye in my .38Spl loads. If I loaded the same amount of BP then I simply would not get the same velocity. 3.6 gns of BP simply does not produce the amount of gas volume needed to push the bullet to the same speed as with the amount of gas created when the Bullseye burns. And with that small an amount of BP in that size of volume I seriously doubt that the amount of gas created by the BP all going off at once would produce a peak that is too high for the gun. But I'd want to see a safe test result to be sure.

 

[zimmerstutzen]

Everything I ever heard is that black powder, as used as a charge in a muzzleloader, burns slower over a longer period of time to create less pressure.

The pressure curve maybe lower, but it is also much longer. In fact, according to lyman's Black Powder report, 80 grains of powder in a 28 inch barrel produces less velocity than the same charge in a 36 inch barrel. So the powder must still be burning and still building pressure even after the ball passes the 28 inch mark of the longer barrel.

Smokeless has a higher pressure spike for a shorter period of time. hence the shorter barrels for smokeless guns.

 

[Foto Joe]

Black Powder burns at the same speed regardless of the ambient pressure. With smokeless the speed of combustion increases as pressure builds which creates more pressure which increases the speed of combustion ad infinum until the powder is consumed or until the pressure is released when the projectile begins its travel down the bore. This is one reason why at indoor shooting ranges there will be unburned powder on the floor in front of the shooting stations and it's also one of the reasons why they won't let folks like is shoot there. They're afraid that we might set fire to it. If everybody shot shot Black Powder the only worry would be ventilation.

 

[Sport45]

Instead the pressure comes from the amount of gasses created when the burn occurs. And simply put it takes a lot more BP to get the amount of gas needed to push a bullet at a given velocity.

This isn't entirely true. 40 grains of black powder produces 40 grains of combustion product and 5 gr of smokeless produces 5 gr of gas. The combustion characteristics determine how fast the gas is produced and thus how much pressure is developed behind the bullet.

35gr of black powder under a 255gr bullet in a .45 Colt case would probably give more pressure and velocity than a full case of H4350 behind the same bullet. (But I wouldn't try the latter just to see.)

Not all smokeless powders are the same.

 

[samcolt1860]

....and like I said so succinctly above, it's the chemistry that makes the powders behave differently. :banghead:

 

[Lunie]

Black Powder Mythology

Can we rename this thread "Black Powder Mythology"?


In short: The reaction speeds (burn rates) *can be* slower for smokeless powders, but they can still produce higher pressures in small arms because they produce more gas for a given reaction rate. Smokeless reaction rates are easily tailored for specific applications, and for us, it is as easy as picking up different types off of the shelf to suit whatever we are loading. Reaction rates for black powder are less variable.

Barrel length: As long is there is sufficient pressure to overcome resistance, increasing barrel length will increase velocity, with BP and Smokeless.



Somewhat briefly:

Gunpowder (BP) and nitrocellulose-based propellants (Smokeless) are both considered to be "low" explosives when classified by response. "Low" means that they "deflagrate" instead of "detonate". "Detonation" is when the reaction rate exceeds the speed of sound in the material and creates a shockwave. "Deflagration" is subsonic combustion (burning).

For transportation purposes both BP and Smokeless can, under specific circumstances, be labeled as Class 4.1 "Flammable Solids". See Title 49 CFR, and keep your migraine medicine handy. (I'm not a legal expert or any other type of expert, but I can read about as well as anyone. There is a Haz Mat table in Part 172, and § 173.170 and § 173.171 may be of interest. )

Strictly speaking, the reaction speed *can be* greater in NC than BP. However, in small arms applications, the reaction speed of NC is typically modified (retarded) to suit particular uses.

Both types of propellants work the same way: They generate large volumes of gas very quickly by deflagration (burning very quickly). In an enclosed space such as you find in small arms, this creates usable pressure for lobbing projectiles. NC propellants are much more efficient in that the vast majority of their reaction products are gaseous. BP is much less efficient, as much of the reaction products are solids (about half, noticeable as fouling).


But don't take my word for it. There is plenty of info on the net, and I think we learn and understand more by researching than by reading someone else's answers or opinions.

 

[Johnson1631]

Blackpowder only burns at one speed.... fast..