bbl length and pressure

[spitballer]

I am trying to understand how powder speed affects pressure, and I wonder if anyone can enlighten me here: if peak pressure is achieved when the bullet is still relatively close to the chamber, can barrel length have any affect on peak pressure when a slower powder is used?

Am I to understand that a slower powder only produces a higher EFFECTIVE pressure, and that because of the longer duration?

Also, it seems to me that that effective pressure might be just as important as peak pressure when considering stress on parts. Would a slower powder at 55,000 PSI cause significantly more stress than a faster powder at the same 55,000 PSI? If so, then wouldn't a pressure figure expressed CuP be more valuable than a pressure figure expressed in PSI when considering a slower powder?

Thanks for any constructive input here.

 

[rcmodel]

The slower powder produces more velocity because it burns longer, and pushs on the bullet longer.
Even if the peak chamber pressure is the same.

Think of it as trying to move a bowling ball.
You can double up your fist and hit it as hard as you can. (Fast powder).
All you will do is break your fist.

Or, you can push it with the same force with the palm of your hand. (Slow powder)
The bowling ball will go much further, faster, and you won't break your hand.

Pretty much the same applies to firearms.
High pressure fast powder has to impart a sharper blow to the firearm to get the bullet moving as fast.

rc

 

[ironworkerwill]

Slower powders work better with longer bbls. If a faster powder is used in a shorter bbl most of the powder is burned before the projectile exits- in theory.

So the slower powder is usually used in the longer bbl of the same cartridge to produce more velocity. Or the slower powder is used on a heavy projectile of the same cartridge to produce more velocity. In theory the gasses produced by the slower powder will contnue to push even as the peak pressure has spiked.

Does this mean that slower powders are more accurate? No.
Faster? Mostly.

By fast and slow I mean in a cartridges relative powder category. BMG will not get good velocity from a .308 and faster powder in that same case ,such as trail boss, will not produce good velocity either.

Most folks want 90+% of the powder to burn before its blown out of the barrel. And that % depends on many varables such as case volume, length of tube, weight of projectile, and diameter of projectile(or dia of tube).



On the topic of speed of and duration of pressure spike: I would think a slower spike is less harmful to metal as metal has a crystaline structure. I know that shocking metal is a good way to weakin it. Quenching hot steel makes the steel much harder but it will break like glass.

 

[jmr40]

Slower powders work better with longer bbls. If a faster powder is used in a shorter bbl most of the powder is burned before the projectile exits- in theory.

No!

Slow powders burn slower and use more of the barrel, but still burn within a very few inches. This might be a concern in very short pistol length barrels shooting rifle cartridges, but not in anything legal as a rifle in this country.

The longer barrels will still generate more speed regardless of powder. While all the powder is burned within the 1st 4-6", the longer barrel will give more time for pressure to work on the projectile increasing speed.

What ever powder you find that gives the best speeds in a long barrel, will also give you the best speeds from a much shorter barrel. At least within reasonable limits.

 

[ironworkerwill]

NO! I guess I don't see whats wrong with that statment?


Pressure spikes within the first few inches.

While all the powder is burned within the 1st 4-6",

The powder, depending upon application and type, does not always.

 

[spitballer]

Okay there seems to be some disagreement, but we all seem to agree that a slower powder will, IN THEORY, propel a projectile faster in a barrel that's long enough to make use of that sustained pressure. This is pretty simple and I can easily understand this.

The point was made that in a shorter barrel, velocity is achieved, IN THEORY, in large measure as a result of powder speed as opposed to duration of pressure. This is still pretty easy for me to understand so far.

However, the effect of duration of pressure has me stumped. Even though expanding gases exert tremendous pressure, they are still gases and It's hard for me to get my mind around the idea that they are instantaneously exerting their entire pressure. It seems to me that even rigid metals have some degree of elasticity and that duration is going to have an impact on effective pressure.

 

[murf]

blow up a balioon. lung pressure expands the balloon evenly on the entire surface. pressure from powder burn does the same thing to the inside of your chamber/barrel. breach face and chamber walls aren't going anywhere, so the bullet takes off down the barrel.

murf

 

[R.W.Dale]

This chart depicting the various port pressures of ar15 gas system lengths is also great for explaining how the powder is fully consumed and pressures peak very early in the bullets travel down the bore.




A faster powder will move that peak pressure point sooner but since you have to use less of it to stay under your maximum pressure the volume of gas produced by the burning propellant is less and therefore bore pressure drops quicker per unit of bullet travel resulting in...get this...less velocity in any bbl length*



*within reason, magnum revolvers rounds in snubs can get outside of this rule

 

[R.W.Dale]

Guns are not rockets. Like folks think when they picture propellant burning the whole time the bullet is going down the bore.

Gunpowder produces gas when burned. It's that gas that drives the bullet. This happens nearly instantaneously after which pressure is continuously dropping as a simple function of the volume it has to fill behind the bullet is continuously expanding.

 

[murf]

nice chart, r. w. dale. wish it showed bullet travel distance to show how far bullet is down barrel at peak pressure.

not agreeing with your conclusion, however. any reloading manual will show faster burning powders can give higher muzzle velocities. depends on a lot more than amount of powder burned, i guess.

murf

 

[R.W.Dale]

, however. any reloading manual will show faster burning powders can give higher muzzle velocities. depends on a lot more than amount of powder burned, i guess.

murf

Bore case ratio and pressure rating dictate what powders will be optimal.

Basically the slowest powder that can still reach SAAMI maximum pressures for a given cartridge bullet wt combo will be the top performer in terms of velocity.

Too slow and you can't fit enough in a case to reach full pressure

Too fast and you're not generating the volume needed for top velocity because you over pressure on less powder

Barrel length is quite literally after this fact

 

[R.W.Dale]

Just look at the pistol length gas system.

Pressure has already peaked AND dropped 10,000 psi a mere 4.5" from the BREACH FACE


(Correct me but I assume the gas port distance is based off ATF measuring rules)

 

[murf]

still, the reloading manual doesn't lie.

how about this line of reasoning:

fast powder has a quicker peak pressure point. that means the bullet acceleration is greater and, therefore, velocity is greater sooner (kind of like a drag car getting off the line quicker). so, at the beginning of the bullet travel down the barrel the quicker powder pushes the bullet faster.

i don't know how much faster for how long, but it seems it is possible that a quicker burning powder can result in higher muzzle velocities.

i'm no ballistician, so i'll leave it at that.

murf

 

[Mad Chemist]

Murf,
Your manual doesn't seem to agree with mine.

The slower burn rate coupled with greater powder density will almost always yield higher velocity. The only exceptions are in very short barrels, <3''.

I think you are getting confused because you are ignoring the density of the powder and only considering velocity. Look at the charge weights in your manuals. Compare those with the volume of the case that they fill.

So for max velocity we are looking for the greatest volume of gas to push the projectile. A denser powder with a lower burn rate will allow for a greater mass of powder to fill the case. This will allow for a greater mass of gas to be produced at a lower pressure than the corresponding faster powder will have produced.

I know I'm over-simplifying this a little, but it holds true for similar powders since their overall stoichiometry for moles of gas produces per mole of powder consumed during combustion are close enough to be negligible.

 

[R.W.Dale]

Look at this thread in which I test a very slow and fairly fast powder for 357 magnum in barrels with a pretty extreme length spread
http://www.thehighroad.org/archive/index.php/t-653813.html

 

[MustGoFaster]

Car guy here, not a great analogy.

Given the same car (power and weight) , a quicker 60 foot time (time to cover the first 60 feet of the track for you non-drag racing people) will result in a slower MPH (terminal velocity) but a quicker elapsed time (time to cover the 440 yards). This is because the car gets pushed by the engine the entire time the car is on course. A quick 60' means you covered the distance quicker, so the power (engine) had less time to accelerate the mass, meaning slower speed at the end.

So, when a car "hooks good" and gets a low 60' time, it makes it to then end sooner but is going slower since the engine had less time to accelerate the mass. A drag racer rule of thumb is that .1 second better over 60', doubles at 1320 (440 yards). A 1.8 second 60' nets you a 11.8 @ 121.8 MPH, a 1.7 60' nets you a 11.6 at 120.9MPH and a 1.6 60' nets you a 11.4@120MPH

A gun (or projectile with compressed gases accelerating it) does not work the same. All the acceleration is taken care of in the first 60' (4 to 30" of barrel) and then the bullet spends the rest of it journey loosing energy instead of having it added.

Fast powder gets the bullet out the barrel in less time, slow powder gets the bullet out the barrel at higher speed.

 

[murf]

mad chemist,

we are talking max pressure, not powder density. fast powders can be more dense than slow powders (blc-2 is more dense than imr7828).

mustgofaster,

greater acceleration equals higher speed (basic physics). maybe a bad analogy, but a steeper pressure curve means greater acceleration and therefore greater velocity. a quicker powder will have a steeper pressure curve. i don't doubt the slower powder will accelerate the bullet at a higher rate for a longer period of time. but this happens later.

anyway, i hope the op is not getting too confused and is getting a few answers to his questions.

murf

 

[murf]

r. w. dale,

do the test with h110 and h4227. h4227 is a slower powder. h110 has a much higher mv at the same max loading. like i said, i don't agree with your blanket statement.

murf

p.s. i was looking in hodgdon's 2011 annual manual, p. 158, 146 gr spr jhp.

 

[ironworkerwill]

Murf, I think he is describing optimal powders. You can use a powder that's too slow or too fast and not achieve optimal results. But comparatively speaking slower powders do produce more velocity.

H414 produces more velocity in 243 than H380 and they have similar densities and are both spherical.

 

[murf]

not with a sierra 60 gn hp bullet. page 82 in the same hodgdon reloading manual.

murf

 

[R.W.Dale]

not with a sierra 60 gn hp bullet. page 82 in the same hodgdon reloading manual.

murf

Bullet mass changes which powders are optimal. Lighter bullets need a faster powder to keep up with the quicker acceleration. Vise versa for heavy bullets

But what doesn't change is that optimal powder for a load that's fastest in a LONG barrel WILL STILL nine times in ten be the propellant that yields the best velocities from a short one.

You get what we're saying. And your hinting yourself at it you just aren't grasping it. Lots of factors combine to determine optimal propellant for a load EXCEPT FOR THE BARREL LENGTH. The factors that determine the loads potential are all sorted out before the bullet even gets halfway down a carbine bbl.

You can have TOO slow a powder just like you can use too fast. Changing the bbl length has no bearing whatsoever on the fact that if the burn rate isn't optimal it's not optimal

 

[Mad Chemist]

bbl length can be thought of as a coefficient that just magnifies the difference b/t the optimum and sub-optimum powder performance.

 

[LeftyTSGC]

I dont know if this will answer all of your questions, but i did do a comparison test of like ammo on different handguns. One with a 3.1 inch barrel and the other with a 4.33 inch barrel. Actual bullet travel is 2.0 inch and 3.3 respectively if i measured correctly.

Look at the chart and you will see that a longer barrel does increase velocity given same load. Thus assuming same pressure up to the length of the shortest barrel lenght.

I am assuming that initial pressure is the same in both chambers until the bullet leaves the barrel and pressure drops, this will be earlier in a shorter barrel, thus producing less velocity.

http://www.thehighroad.org/showthread.php?t=752385 See chart here

 

[spitballer]

Concur with the thanks to R.W.Dale for the chart! Speaks volumes!

R.W. D: Okay I follow pretty much what you're saying about the faster powders and the limits that must be imposed on them because of their tendency to produce excessive pressure at the beginning of the curve.

But let's say I'm using a volume of a fairly fast propellant that fills the case to capacity, like IMR 3031. Would it produce a curve similar to a curve produced by a similar speed powder that only fills the case partway like, say, an Accur 2015?

Let me try to illustrate using a combustion chamber in a gas engine: Wouldn't the smaller starting volume of propellant be like a lean mixture that, despite it's theoretical burning speed, can't produce the VOLUME of burned gases necessary to actually move the piston properly? And wouldn't that smaller amount gas thus have more elasticity within the confines of the chamber and barrel?

Certainly the density of the powder would account for something, but both the 3031 and the 2015 weigh around 13-14 grains per CC.

It's interesting to note that the max load of 24.6 grains of IMR 3031 only produces 52,500 or so PSI, whereas that same 24.6 grains of Accur 2015 would produce somewhere in the neighborhood of 67,000 PSI!