The .220 Swift has a case capacity about 10% larger than the .22-250.

 

I understand that, but it has lower pressures and sometimes higher velocities. Why does the 22 250 have higher pressures and slightly lower velocities?

 

Peak pressure by itself doesn't determine the velocity. You need to know the shape of the pressure-distance curve.

 

Capacity means velocity because it's burning more fuel.

 

True. You also have to consider the effect on the expansion ratio. The 'real' one being measured from propellant burnout.

 

I'm thinking that because the Swift was made standard many years ago, it was assigned a lower maximum pressure. The 22-250, by contrast was standardized in the late '60's.

 

If I am reading correctly, when reloading the shells with the same powder, the 22 250 shows that it has higher pressure.

 

You can't compare the same powder in cases of different capacities. The burning characteristics change with volume and bore diameter as well as bullet weight, temperature and phase of the moon. The Swift operates best with slower powders because it has more capacity. The 22-250 AI is very close in capacity and in velocity and the AI likes the same powders as the Swift.

 

...and phase of the moon.

Here is a list of required parameters.

 

I haven't seen any data on the 22 250ai. Does the pressure drop to the same level as the swift since the case is enlarged?

 

Same volume gives same pressure IF all else is the same. If you load a 22-250 load in an AI chamber, the pressure stays down and velocity stays about the same. If you load a Swift load in an AI chamber you get the same pressures and velocities as the Swift. (NOT saying that's safe.)

Bigger case equals lower pressures OR a bigger case holds more powder and gives the same higher pressures.

If the goal is velocity, pressures will be high.

 

Ok. Thanks for being patient with me. Lol.

 

MAN! Am I glad to learn that! I always wondered.

 

Standard high school math except the "Z". That one puzzles me.

 

That's easy.

Z = 1 - (1/2)(g - 1)M + (1/2)(g - 1)Mf
g = gamma

 

Ok, so 1 plus one is 2?
That stuff is above my pay grade!

 

That gamma thing scares me. I had enough of those shots in Korea.

 

That gamma thing scares me.

Not to worry!

 

Alls I know is a squared plus b squared equals c squared or 3+4=5. No gammas or z's in that one.



RJ

 

Alls I know is a squared plus b squared equals c squared or 3+4=5.


RJ

Close!

3^2 + 4^2 = 5^2

 

Both old school equations, F2G1D, but I like the way you put them up on the board for all to see. Veteran stuff there...

 

9+16=25 :d

RJ

 

Probably a good place to get a feel for what is going on in a gun barrel, in terms of pressure changes as the bullet moves through the barrel...

Is an introductory text book to heat engines, you should be able to get one as a free .Pdf download from archive.org something like Ripper's "heat engines" his book " steam" is a bit more advanced than you'd want for an introduction.

The bit which is directly relevant is "indicator" or "pressure-volume" diagrams. https://en.m.wikipedia.org/wiki/Pressure_volume_diagram

They allow you to graphically compare the pressure- bullet travel ( which is the same as pressure: volume) with the ideal Otto and Carnot cycles and to understand how changes in burning rate of powder, case volume, peak pressure etc, affect each other and affect bullet velocity and energy.

The equations are mathematical representations of those diagrams.

The shape of a case is largely irrelevant to this, unless you have extremely long skinny cases, say a 12 inch long straight sided .22 case, where heat loss to the case walls might be getting fairly significant. For all reasonable cases, we can assume that the case just acts as a container for powder and that case has a known volume. That case volume is important.

 

It's sure hard to overcome the 'magic shapes and venturified shoulders' of the advertizing world.

Capacity determines performance, Shape determines feeding and the rim determines extraction. The rest are details... like a new name.

 

Capacity determines performance...

Shape is irrelevant for purposes of calculation. In fact, the initial airspace is simply imagined to be a cylinder.

 

Thank you! I actually kinda understand that!
So, it's the amount of powder burned that has more influence on the throat erosion more so than the pressure difference?

 

“Erosion is a highly complex phenomenon which involves the combined

interaction of thermal, chemical, metallurgical, and mechanical factors.”

Physicochemical Aspects of Gun Barrel Erosion and its Control, Ahmad

 

Pa Pap,

The bullet is being accelerated any time there is pressure behind it. While there is greatest pressure at the peak, that usually occurs when the bullet is only one or two inches into the bore. As the bullet goes further forward, the pressure behind it decreases, but not to zero, so it keeps accelerating, picking up speed, but just not as rapidly. In the gun with greater case capacity you burn more total powder so you make more total gas. Also, the volume of the bore is a smaller percentage of the case volume, so the percent increase in total volume containing the gas when the bullet reaches the muzzle is less for the 220 Swift. As a result of the combination of more gas and less expanded volume, pressure is higher at the muzzle of the 220 Swift than with the 22-250, so the acceleration of the bullet further down the barrel than the peak was greater and giving the bullet more velocity.

Ignoring friction for convenience's sake, muzzle energy is proportional to the average pressure that was behind the bullet for its whole trip down the barrel. This is because muzzle energy equals work done on the bullet, work is the distance the bullet travels in the barrel times the average force behind it, and average force is equal to average pressure times the bullet cross-sectional area. That average includes start pressure, peak pressure, muzzle pressure and everything in between them. In the 220 Swift, the peak may not always be as high as the 22-250, but the muzzle pressure will be higher for the reasons I already explained, and, taken together with the pressures in between, it is enough to provide a greater average pressure than the 22-250 can, hence the greater muzzle energy and the higher velocity responsible for that.

 

Thanks for the explanation.
I guess the reloading books give the highest peak pressure of a given cartridge but not necessarily the overall energy developed by the cartridge.

 

They give you the energy retained by the bullet at the muzzle, if that's of interest. That's not the same as how much energy was in the propellant burned, of course, but it's useful.

 

I guess the reloading books give the highest peak pressure of a given cartridge but not necessarily the overall energy developed by the cartridge.

Correct. Total energy produced is a whole 'nother can of worms and has many more variables to it, but it boils down to what propellant and how much of it.

 

For practical purposes the 220 Swift and the 22-250 both generate the same maximum pressure, 62,000 psi.

They have different chamber volumes. This means that either will have different optimal powders. Comparing performances based on equal charges of the same powder is not a appropriate comparison.

 

Pa Pap, no use to ask for the reason as with the SAAMI advice there is no logically evident reason.

*Ask why the .270W is advised for 65,000 psi but its parent case is proposed to be much lower?

* Ask why the 7mm Rem Mag and .300 Win Mag are being advised to be loaded to lower pressure than the .270W?

*Ask why the .280 Remington had from the very beginning been downloaded to silly levels? Even now it is still "limited" to 60,000 psi but in no reloading table will you see loads to even this figure.

One may speculate about the reasons for the following: Despite its up rated to 65,000 psi allowance the .270W barely nudges the performance of the .308 Winchester with similar weight bullets and does not have the design ability to use the heavier bullets which allows the .308W to outshine it on heavy game. As it is, its parent cartridge, the 30-06 outperforms its trajectories with a considerable margin, and doing so even with heavier bullets. Had the .270W been limited to 62,000 psi like the .308W nobody would even have looked at the .270W due to its distinct lack of similar ability at that pressure.

So,there is no logical reason, despite the mathematics displayed above - none of which directly addresses any official reasoning. SAAMI had done some tests and arbitrarily stopped the tests and declared a maximum pressure, and not by applying the above mathematic equations which had been derived from set geometric shapes.

Consider this as the only reason left after all the logics had been depleted and found lacking in substance: SAAMI is owned by the very industry it proposes standards for instead of being an independent standards bureau like every other nation has, and therefore it is open to lobbying and negotiation. The old concept of creating marketing appeal is but one evident driver in the decisions on pressure advice for certain cartridges...

 

...the .270W barely nudges the performance of the .308 Winchester with similar weight bullets ...

Something to consider is the 270's greater initial volume , but its lesser increase in volume per inch of travel(smaller bore area). An interesting observation is the Bore Capacity Index I created: