Barrel length verses powder burn rate?

[adams484]

I am hoping to get some input on powder choices in relation to barrel length.

I am interested in handgun loads and the timeline in the milliseconds it takes to consume the propellant.

With that said let's review some of the things I have found so far. Some of the threads I have read on this subject stated that barrel length is irrelevant to propellant burn rate.

If that were true then why does barrel length affect velocity?

Is it because the pressure stays behind the bullet longer? If so then what happens when the propellant is still burning when the bullet leaves the barrel? Flash? Powder/smoke screwing with your chronograph readings?

It just seems that short barrels like 2 inches or less would benefit from a faster powder loaded to max pressure because a slower powder might not be all consumed before the bullet leaves the barrel. I even heard that even slow burning powders are consumed in the cartridge but I am doubting that as that would make the difference in velocity of longer barrels inconsequential or hardly noticeable. I wish I knew the consumption rates in terms of milliseconds of the powders as it would be easy to calculate or perhaps easier with some calculus on the FPS and acceleration of the projectile.

Sorry if I have gone over the top with this but time is money and the cost of determining the best powder for the job empirically seems like it could be refined a little with some simple rule sets that I have yet to find or determine. Can any of you out there add to this line of thought other than just use a recipe and shut up.

 

[Rule3]

Slower powders are "generally" preferred to obtain more velocity out of short (snub nose) barrels.

 

[adams484]

Slower powders are "generally" preferred to obtain more velocity out of short (snub nose) barrels.

I have heard that but it just seems counter intuitive but a lot of thing are in life. I have seen slower powders produce more flash at the barrel. Particularly magnum loads which predominately use slower powders. It just seems like a waste of powder to burn it in the air instead of behind the bullet.

 

[jstein650]

edit: I have focused more on rifles to illustrate, but the same principles apply to handguns.

This has been addressed, and frankly the actual internal ballistics can be a bit hard to know. Unless you're looking real extremes, a longer barrel will always give higher velocity since the gas pressure behind the bullet will have more time to exert a force to accelerate the bullet. There is a point of diminishing returns when you get into really long barrels - many feet, and this has been demonstrated in the lab. In other words, the gas pressure is still much higher than needed to overcome barrel friction in most any load/barrel. When a charge of fast powder is burned, the breech of the barrel will have to contain virtually all of the pressure (and the steel has a limit), and the bullet will have to accelerate as it does, and that gas pressure will drop as it has more bore volume behind the bullet. A slower, more progressive burn will continue to push on that bullet as it begins to accelerate; longer harder push. This is essentially what the new Superformance powders do to get higher velocities. Whether or not all of the powder is consumed is really not so much the issue in the real world of rifles anyway. The dynamics of the push in the first few inches of barrel still trumps overall powder consumption efficiency. I'm sure it's quite complete in a 12ga, for example, but that's an extreme case of large bore, small charge, and fairly low velocities. As to how short a rifle barrel would you have to have to negate the velocity advantage of the slowest appropriate powder? I don't know, but I'm thinking pretty short, regardless of the ball of fire coming out of the end.
All of that said, in extreme cases, like a very short barrel handgun, something less than the slowest appropriate powder is usually preferred to reduce a huge flash.

 

[Ken70]

Another factor is bullet weight, 125 grain 357magnums have a huge fireball. 158 grain, not so much. Lighter bullet moved faster and exited while the powder was still burning. Lots of fun at night...

 

[ranger335v]

"Is it because the pressure stays behind the bullet longer?"

Yes.

 

[morcey2]

All smokeless powder has as complete a burn as will happen before the bullet has traveled significantly into the barrel. It's on the order of tenths of milliseconds. The muzzle flash you see isn't still-burning powder but hot, incandescent gases. The powder that gives highest velocity out of a 26" barrel (thinking rifles here) will give the highest velocity out of a 16" barrel. More slow powder (within reason and pressure limits) will generally give higher velocity but will also increase the muzzle flash/blast. It applies to handguns, but at a smaller scale.

Matt

 

[Rule3]

Well if you factor all other variables such as barrel length, flash, bang, bullet weight and the like it is impossible to answer.

But for the highest velocity, lets say with a 158 grain bullet out of a 2" snub nose 38 special, a slower powder will given you more FPS at the barrel. Pressure builds up slower and creates the faster FPS, before it is all gone out the barrel.

Very easy to document using a chronograph and different powders. I am sure there gazillion threads on the subject

 

[rcmodel]

It just seems like a waste of powder to burn it in the air instead of behind the bullet.

You aren't burning it outside the barrel.

Slower powder retains pressure in the bore longer, and the resulting muzzle flash is the result of higher pressure white hot gas coming in contact with free oxygen in the air.

Regardless of the powder choice, it all burns in the case, chamber, and first inch or so of the barrel.

Slower pistol powder gives higher velocity then fast powder because it maintains bore pressure longer and continues to accelerate the bullet longer.

But the fact remains.
Whatever powder gives the highest performance in a long barrel will also give the highest performance in a short barrel.

rc

 

[jstein650]

rcmodel:

I agree with your conclusions, but,

"You aren't burning it outside the barrel. ... muzzle flash is the result of higher pressure white hot gas coming in contact with free oxygen in the air."

You're saying the stoichiometry of nitrocellulose/nitroglycerine combustion leaves a residual combustible in the presence of atmospheric oxygen? I always thought such explosives where made with an excess of available oxygen in the compound.

There are a lot of additives, though, (not to mention bullet jacket residue) and that makes a compelling explanation for some pretty heavy (huge) muzzle flash that I've witnessed.

 

[jerkface11]

No it isn't burning when it comes out of the barrel. It's HOT. Hot things glow.

 

[rcmodel]

You're saying the stoichiometry of nitrocellulose/nitroglycerine

I don't know big words like that.

It's HOT. Hot things glow.

That is how I would have to explain it too!

rc

 

[1SOW]

You're saying the stoichiometry of nitrocellulose/nitroglycerine combustion leaves a residual combustible in the presence of atmospheric oxygen? I always thought such explosives where made with an excess of available oxygen in the compound.

Sort of: Superheated paricles from "burning" and being compressed are exposed to more oxygen when leaving the bbl.

It's also interesting that "single based" powders composed of only nitrocellulose, don't leave enough superheated particles to put on the light show. The particles burn more efficiently with less residue/cleaner. They also tend to burn cooler. VV comes to mind.

 

[jstein650]

Quote:
It's HOT. Hot things glow.
That is how I would have to explain it too!

That works for me.

 

[morcey2]

It's HOT. Hot things glow.

I keep telling my wife that's why everyone averts their eyes or throw on their sunglasses when I walk by in shorts.


I'm not allowed to wear shorts anymore. Court order.

Matt

 

[jstein650]

HA! Stop it...

 

[gamestalker]

I load exclusively with slow burning powders in all my weapons, rifles, handguns, short barrels and long. An example of the difference between a long barrel and short isn't really discernible from a visual perspective. It's either a flashy powder, or it isn't. Comparing a 6" handgun barrel with the same load in a snubby really seems to produce about the same degree of flash, at least from what I can tell.

As for M.V., I see an increase that correlates with longer barrels vs shorter one's. There seems to be far more correlation though with long guns. And yes, it is because the pressures are behind the projectile for a longer period of time that velocity increases. From what I understand, powders are designed to burn completely, or produce the same peak pressures regardless of the barrel length. What I don't quite understand, is how a projectile that leaves a chamber in the cylinder, and then travels across a gap where much of the pressure is leached off, can still produce higher velocities with a longer barrel? Maybe someone else with more knowledge of internal ballistics can clarify why / how.

GS

 

[adams484]

All smokeless powder has as complete a burn as will happen before the bullet has traveled significantly into the barrel. It's on the order of tenths of milliseconds. The muzzle flash you see isn't still-burning powder but hot, incandescent gases. The powder that gives highest velocity out of a 26" barrel (thinking rifles here) will give the highest velocity out of a 16" barrel. More slow powder (within reason and pressure limits) will generally give higher velocity but will also increase the muzzle flash/blast. It applies to handguns, but at a smaller scale.

Matt

This validates the idea that the powder is consumed in the cartridge or near it and incandescent gasses makes a lot of sense.

Another thing to consider would be pressure as some powders need more pressure to completely burn.

 

[Jim Watson]

I always thought such explosives where made with an excess of available oxygen in the compound.

Not the case.
Nitrocellulose and nitroglycerine do not have enough oxygen to completely oxidize the carbon and hydrogen in the propellant to CO2 and water. So you get some discharge of hot hydrogen and carbon monoxide into the air.
Now why this hot gas ignites and flashes brightly in some cases and not in others, I don't know. But it is observable.

 

[adams484]

Comparing a 6" handgun barrel with the same load in a snubby really seems to produce about the same degree of flash, at least from what I can tell

Ahh... A way too prove the theory... I will do exactly that with a particularly flashy powder and a not too flashy one comparing a S&W 686 4" and a 2" snubby. It makes sense that if the flash is only incandescent gasses then there should be little difference in barrel length to flash. But if it is unburnt propellant then the difference should be visible/recordable.

 

[rcmodel]

But if it is unburnt propellant then the difference should be visible/recordable.

Not necessarily.

The residual bore pressure at the muzzle will still be less in a long barrel.

It's just lower, but still pushing on the bullet.

But it's not full chamber pressure, or even as much as it is 2" into the barrel.

rc

 

[roadcoder]

You shouldn't even be thinking about stuff like this. Just enjoy your gun, for goodness sakes!