Case Capacity Question

[jwrowland77]

I have a question concerning case capacity as it correlates to pressure and MV.

I weighed two different headstamps tonight for .308 win....Winchester and Lapua. The Winchester had a average case capacity of 56.8gr. The Lapua brass has a average case capacity of 55gr a difference of 1.8 grains. The Winchester brass was the lightest of the two empty.

Now, when I did my workup in each brass case, my best load in the Win brass was 44.5gr of IMR4895 and in Lapua brass it was 43gr of IMR4895. Does this normally happen where the difference between loads worked up are close to the same in the difference of the case capacity?

I don't have a chrono...yet, but have a question about pressure and muzzle velocity.

With the Lapua 43gr charge, and the Winchester 44.5gr charge, would these have the same or close to the same pressure and/or same or close to the same muzzle velocity even though one had a lower grain charge, since the difference is in case capacity?

Just something I was sitting here thinking about as it might correlate to MV and MOA adjustment might or might not be the same.

Thanks,
Jeremy

 

[918v]

Velocity has nothing to do with pressure. It has everything to do with the amount of powder you load in the case. Case capacity affects pressure, so you may not be able to use the amount of powder you want to reach your target velocity at a safe pressure level.

Winchester cases will let you use about 2grains more powder than Lapua before your bolt gets sticky. This translates into higher velocity, but how much depends on barrel voodoo.

 

[Bart B.]

If you're weighing a case full of water, note its capacity will vary depending on how much out of round it is as well as its headspace length (head to shoulder) compared to others. If the fired primers are still in, their weight will effect the numbers you get; very tiny amounts, but it's there.

I prefer just weighing empty cases. Each one in a lot of cartridge brass is very uniform in its metalurgy. A different lot may be different and weigh a tiny bit different even if their dimensions are exactly the same. They're all pressed hard against the chamber walls and bolt face at peak pressure and all cases of the same lot and weight will all have the same volume inside at that time.

Unless you shoot your stuff into no worse than 1/4 MOA at 100 yards, sorting cases to a 1% spread in weight's plenty good enough. Sorting to half that spread may reduce your groups by 10%.

I've loaded equally accurate .308 ammo in cases with 150, 157, 168 and 172 grains changing only the powder charge weight; all other components were the same. I doubt average muzzle velocity spread for each of the 4 case weights was more than 70 fps, but I didn't care how fast they left the barrel. All I wanted them to do was land on target very close together.

I believe velocity has a lot to do with pressure for a given cartridge. Having shot the same .308 Win ammo with .3082" diameter bullets in two 26" 4-groove barrels, one with .3084" groove diameter and the other with .3078" groove diameter, muzzle velocity and peak pressure were both higher with the tight bored barrel evidenced by slightly flatter primers on its fired cases and higher numbers on the chronograph.

 

[brickeyee]

"Velocity has nothing to do with pressure."

Say what?

The velocity of a bullet is the integral of the pressure down the barrel.

Peak pressures are often so brief they have relatively little affect on final velocity (the area under that small portion of the pressure-time curve is not all that large compared to the total area for the whole barrel time) but pressure is exactly what makes high velocity possible.

Peak pressures blow things up, average pressures determine bullet velocity.

 

[GJSchulze]

Velocity has nothing to do with pressure

Say what?

That was my first reaction to 918v's statement.

Smokeless powder is not an explosive. It burns really fast and produces a lot of gas. Expanding gases confined in a small space produces pressure. This expanding gas pushes the bullet out the barrel. Pressure correlates to and is causative to velocity. Other factors that affect velocity is seating depth (or OAL) and barrel length. OAL made shorter, decreases case volume, which increases pressure at ignition resulting in increased velocity. A longer barrel increases velocity because the gas has longer to push on the bullet.

Now 918v says that it's the powder charge that has to do with velocity. That's certainly correct. 918v may have meant that the only direct thing a reloader can do to change the velocity is to change the charge and that we, as reloaders, without very expensive equipment, cannot measure the pressure. You can use Quickload or similar program to calculate the pressure and to display a graph of the pressure curve.

 

[Walkalong]

I would not go so far as to make a blanket statement that pressure has nothing to do with velocity, because it does, but there are other variables to consider, and two different loads with the same peak pressure can produce different velocities. It has to do with burn rate and charge weight/volume.

 

[GJSchulze]

I would not go so far as to make a blanket statement that pressure has nothing to do with velocity, because it does, but there are other variables to consider

Isn't it safe to say that the pressure is what causes the bullet to move? More accurately, pressure a measure of the expanding hot gas that moves the bullet.

Would it also be safe to say that all of the variables that affect velocity can be summed up in the shape and amplitude of the pressure curve? The burn rate certainly is included in that as well as the peak pressure. Everything that we can do to affect velocity is in the pressure curve: powder, powder weight, seating depth, bullet weight, case volume, bullet profile (because friction on the barrel affects pressure), and primers (if you think they make that much of a difference). All of those are what produce a particular pressure curve.

Pressure is to expanding gases in a barrel as pressure is to water in a hose or voltage is to current.

 

[918v]

Pressure correlates to and is causative to velocity.

No.

You can increase pressure without increasing velocity.

Velocity is caused by the energy of the combustion of powder, not by pressure.

 

[Walkalong]

Velocity has nothing to do with pressure

Simply cannot be true.

Instead of drive by blanket statements, care to explain your thoughts?

 

[918v]

I did explain them.

 

[USSR]

Isn't it safe to say that the pressure is what causes the bullet to move? More accurately, pressure a measure of the expanding hot gas that moves the bullet.

Yes, you have it. This argument is like arguing that it's not food that makes people fat, it's the calories.

Don

 

[918v]

If pressure caused velocity then you could reach your target velocity with any powder. All you would have to do is work up to whatever PSI you needed. But this isn't the case, is it?

 

[USSR]

If you're talking about using a fast handgun powder in a rifle case and loading it up to the same pressure level as a normal rifle powder, then no. But we are talking about simply increasing the pressure with components that are normally used for the application.

Don

 

[jwrowland77]

If pressure caused velocity then you could reach your target velocity with any powder. All you would have to do is work up to whatever PSI you needed. But this isn't the case, is it?

I was thinking that if you're using different powders same headstamp, then no. However, using same powder, but different headstamp swith smaller case capacity then yes.

I am hitting the same POI, same powder, different stamps on cases, just a smaller charge in case with smaller capacity.

Just my thinking. I have been enjoying the conversation on this thread. It's given me lots to contemplate.

 

[918v]

If you're talking about using a fast handgun powder in a rifle case and loading it up to the same pressure level as a normal rifle powder, then no. But we are talking about simply increasing the pressure with components that are normally used for the application.

I'm not talking about using a fast handgun powder in a rifle case. That is a strawman argument you designed.

 

[918v]

I was thinking that if you're using different powders same headstamp, then no. However, using same powder, but different headstamp swith smaller case capacity then yes.

Same powder, smaller case capacity may not allow you to reach the same velocity at the max pressure level. You read about people hitting crazy velocities with the 308 but they're using Winchester cases and two to three additional grains of powder. It is the additional capacity that allows them to use more powder within the safe pressure limit. Everything else being equal, the higher capacity case has more velocity potential at the same pressure limit.

 

[Walkalong]

But "Velocity has nothing to do with pressure" is still false. It's not the only thing that has to do with velocity, but it does have something to do with it. It takes a volume of powder operating at a certain pressure to get a certain velocity.

 

[918v]

Pressure is only a byproduct of the combustion process. You can achieve the same velocity at completely different pressures using different powders. You can increase pressure by reducing case capacity and gain nothing in terms of velocity.

 

[USSR]

You can achieve the same velocity at completely different pressures using different powders.

How profound!

Don

 

[Andrew Leigh]

OK lets throw out the excessive variables which will cloud the discussion.

Given Case A with Powder A and all other variables the same.

- A change in powder charge will change pressure as there will be either less or more combustable mass that needs escaping.
- Less combustable material will result in less pressure, less pressure will result in reduced speed. The converse is also true.
- It then stands to reason that altering the internal volume by substituting Case A with Case B that the pressure will change in relation to the change in volume.

The above is my personal reloading experience and is supported by reloading logs.

To change the rules and introduce other variables like different powders with different rates of combustion is perhaps unfair. Yes these will alter matters as powders may achieve the same pressure but at different speeds. It is for this reason that we are able to use different powders and charge weights for the same set of components.

Combustable volume is crucial to speed, that is why we neck down 30-06 and .375 case into wildcat calibres. We attempt to produce as much combustion as possible in order to generate as much speed as possible. We tweak this through finding powders with burn rates that best respond to the components in use.

I think that the OP was referring to all components being the same BUT for the case internal volume. Given this then yes he would need to adjust powder charge accordingly as the pressure WILL change. Given this, for all intents and purposes, if both loads reach the same speeds with diffent powder weights then tis will be indicative that the same pressure is being generated.

But his last statement is partially true. MV does not correlate to MOA accuracy as one can have more than one accuracy node in a sequence of reloads. The "correct MV i.e. read accuracy node" will result in the best accuracy for the firearm.

 

[jwrowland77]

Andrew hit it on the head...all components the same except for the case.

Win vs Lapua. Reduced charge in the Lapua case should give me the same pressure and MV as the higher charge in the Win case. Which wouldn't it stand to reason that they would have the same or close to the same MV since both were hitting the same POI?

I had simply noticed that the reduction in charge, that was hitting the same POI, kind of correlated, or at least it seemed it did, when I weighed the cases and found the the case capacity difference was about the same in the reduction in charge. Both charges, the one the the win case and the reduced one in the Lapua case, almost matched.

Case capacity difference was 1.8 grains between win vs Lapua, and the charge difference was 1.5 grains, and both loads hit the same POI.

 

[Robert101]

I too believe pressure and velocity are directly related. But to keep an open mind on the topic, let’s change the cause of the pressure from an expanding gas to a pressurized liquid. It is true that viscosity, compressibility, and plasticity of liquid may directly affect velocity (in a pressurize tube with a projectile on one end). Sort of like comparing a thick blob of liquid that might exert the same pressure (as a thin low viscosity of a liquid) but the results of each cause very different velocities on the projectile. That blob of liquid might only throw the projectile a foot, but a low viscosity liquid that is very compressible might increase velocity exponentially. I do see 918v’s point but still believe, in the context of appropriate powder relating to a specific caliber, that pressure is related to velocity.

 

[GJSchulze]

You can increase pressure without increasing velocity.

918v, when I was talking about pressure I said the pressure curve, not peak pressure. It sounds to me like you are considering peak pressure and not the whole pressure curve, meaning pressure at each stage of the bullet's travel down the barrel.

About what moves the bullet. The combustion creates gas and this expanding gas is what pushes the bullet. Consider a pea shooter or a blow gun; it's your breath, not the energy expended in making your diaphragm move. Rockets move because of the fuel burning creating hot gases that shoot out the back. Force = Mass x Acceleration. The mass in this equation is the gas produced by burning.

What I'm not saying is that the powder and it's burn rate don't affect the velocity. They certainly do, but do so by how fast and how much gas is produced. The most effective way reloaders have to alter the velocity is with the powder. Case volume, OAL, and barrel length have less effect.

If you still doubt us, please find some other sources to check with.

 

[918v]

Win vs Lapua. Reduced charge in the Lapua case should give me the same pressure and MV as the higher charge in the Win case. Which wouldn't it stand to reason that they would have the same or close to the same MV since both were hitting the same POI?

Define close? 50? 100 FPS?

I think you need to chrono the loads side by side.

 

[jwrowland77]

Define close? 50? 100 FPS?

I think you need to chrono the loads side by side.

Well, yeah. Plan on it, as soon as I can get one.

This was just something I was thinking about. As stated, I don't have a chrony, yet. It just came to mind that if they were hitting the same POI, wouldn't they have to be close in MV? Slower, they would impact lower. Faster, higher? Just trying to learn. Trying to use a little reasoning and trying to figure things out is all prior to getting a chrony.