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A question for the ballistic scientists, err, Wildcatters.

 
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  #1  
Old 12-01-2005, 02:12 AM
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A question for the ballistic scientists, err, Wildcatters.

Every once in a while I hear someone mention that a case design is more efficient or less efficient than another. A 22 K Hornet is more efficient than a .22 Hornet. A Gibbs design is more efficient than an Ackley (or vice versa). Well what exactly does this mean? How does the shape of a case change the efficency? (The powder burns and pushes the bullet down the bore. What does the powder care what shape container it's in?) What benefit do I get from an efficient case?
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Old 12-01-2005, 05:00 AM
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Re: A question for the ballistic scientists, err, Wildcatters.

To answer the last question, with a case with greater efficiency, you get more for less.

Look at the 300 WSM and 300 WM.

190 gr SMK

Both will shoot the bullet at 2900 fps, but the WSM will do that with 60 gr of powder, but the WM will take 68.5 gr of the same powder to reach that velocity. A better example may be 6mm BR and .243 or 6XC and 243. The latter case, both bullets will puch 100 gr bullets to 3000 fps; however, the 6XC does so with 7% less powder.

With a more efficient case design you in essence save money on components and in theory get better barrel life. The last part depends on how you actually use it.
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Old 12-01-2005, 05:22 AM
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Re: A question for the ballistic scientists, err, Wildcatters.

Efficiency due to the shape of the case is largely overblown IMHO. A 300 Win can easily run 190's well over 3000 from a 26" barrel to add to the above example.

The big difference you'll see is simply a function of case capacity for a given bore. A .308 is more efficient (ft-lbs energy of the bullet/amount of energy contained in the powder charge) than a 300 WSM ever will be. The WSM is more efficient than the RUM. The RUM is more efficient than the 30-378, etc.

You can make the cases short and fat, tall and skinny and yes, things will change ever so slightly...but it's a small effect compared with the case capacity to bore ratio.
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Old 12-01-2005, 09:48 AM
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Re: A question for the ballistic scientists, err, Wildcatters.

I to do not feel case design is as critical to efficency as it is made out to me. Sure a shorter fatter powder column will produce better primer blast saturation in the powder but I have yet to see where one case design would produce a significant advantage over another with same length barrels, same pressure and same bullets used.

Barrel variations, throat variations, chamber variations will effect final velocity results much mroe then anything to do with case design.

Now, there is a reason why the comp world is going toward the short fat case design and I personally feel it is because they are more consistant in velocity, not more efficent.

Lets look at the 300 WSM and 300 Win Mag comparision. First off if we put them both in 26" barrels, the WSM has an advantage right from the start as it has more usible bore length for the bullet to gain velocity simply because the shorter over all length of the round. Over 1/2" more usible barrel. Will this make alot of difference, not but it will make some difference.

If you load the 300 WSM with a 180 gr bullet to 60,000 psi and drive the bullet down a usible barrel length of 24" and then load the 300 Win with the same 180 gr pill to the same 60,000 psi with the same 24" of usible barrel, that meaning the length of barrel from the ogive to the muzzle, the Win Mag will produce more velocity every time as long as barrels are of similiar quality.

Just my opinion.

I think the term efficent gets confused with consistant when talking about case designs. For consistancy the short fat cases do have an edge in my opinion but not for efficency, at least how I define it.

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Old 12-01-2005, 12:57 PM
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Re: A question for the ballistic scientists, err, Wildcatters.

Good question 4kie.
I agree with some comments made and disagree with others and here's why:

In reality, there is no substitute for cubic inches in horsepower to attain acceleration just like there is no substitute for case capacity in velocity. However, there are ways to make a smaller engine (smaller case) seem like it becomes greater than the sum of their parts.

It is a well known fact that PO Ackley invented his designs to make use of abundant supplies of mil-surplus powders of the slower burn rate variety. He quickly found out that a sharper shoulder combined with less case taper would disproportionately increase the velocity of projectiles. It was later discovered by Mr. Barnes that super heavy-for-caliber bullets would even more dramaticly take an increase in velocity with this design, thus becoming more efficient.

So, we could take a case (x) and improve it and it would give the same velocities as case (y) which was not improved but had slightly larger case capacity.

I will give you two examples from my personal testing of this phenomenon.

I once had a 220 swift, and a 22-284. I also had (and still have) a 22-250 improved. One day I decided to see how fast the swift could push a 80 grain MK. The case capacity of the swift was 2 grains more than my 22-250 improved, but it only got within 100 fps of the improved using 2 grains more of the same powder. So, the swift used 2 grains more powder to be 100 fps slower than the 22-250 improved (barrel lengths were same).

On the other hand, the 22-284 used 13 grains more powder and had a 4" longer barrel and only shot 80 grain Mk's 200 fps faster than the 22-250 improved! So it used a huge percent more powder to get a relatively small increase in velocity considering the 4" longer barrel. I believe the actual number I have read was for every 6% increase in powder you get a 3% increase in velocity.

The reason it was so inefficient was because there was too much powder to go down the bore in the form of gas after the bullet had already exited the barrel. This is has to do with a term called <font color="blue"> expansion ratio. </font>

The expansion ratio's definition is this: (n.) The ratio of the volume of the bore from the cartridge base to the muzzle, to the volume of the powder portion of the cartridge.

<font color="red"> The higher the expansion ratio, the more efficient it will be. </font>

If you caught this idea, you would realize that a longer barrel increases the ratio and a larger case capacity decreases it. So you can see that even though the 22-284 had a 4" longer barrel, it was not enough to increase the efficiency versus the bigger case! WOW, this is getting deep. I apologize if this is boring you to death!

Ok, now that we understand the effeciency ratios, we can look at another fact that Kirby mentioned ealier. Guns are not terribly efficient machines. Typically, even the most efficient cartridges use 50% of their total potential energy to propel the bullet. Some are much less. A 7mm ultra mag is more like 30%. Kirby mentioned differences in barrels and roughness and other factors where efficiency is lost due to thermal conductivity and thermal dynamics. These are matters also to take into consideration.

Back to the examples I was giving. The other comparison I would like to point out is the 7mm mag, 7mm short mag wildcat, 7mmrhb, and the 7mmultra.

The short mag I have personally witnessed is one of Kirby's guns he did for Brian B. It is a 270 WSM necked up to 7mm. THis cartridge uses the similiar amounts of powder as a standard 7mm mag (65 grains of RL22 with 160 grain bullet) yet yeilds velocity increases of 200 fps over the standard 7mm mag with equivalent barrel lengths! Why? It goes directly back to the efficiency in the case design.

Now take a look at the 7mmrhb. It is a 300 ultra mag necked down to 7mm and shortened. So it has the same diameter powder column as the 7 ultra, but it is shorter. In testing, it showed velocities in the 3600 fps range with 140 grain bullets. THe 7 ultra on the other hand used more powder to give slower velocities in the neighborhood of 3400-3450!

Now, all of these cartridges are capable of being consistent in velocity spreads. In fact, using the right powder in each resulted in several different loads that had deviations less than 10 feet per second. <font color="purple">So, we can see that while an efficient cartridge may also be consistant, a consistant cartridge may not always be efficient. </font> The terms are somewhat related, but are not interchangeable.

Standard deviations in velocity measurements are results of many things, but it has been proven by a host of cartridges that shorter, fatter cartridges are more prone to exhibit low deviations <font color="red"> on average! </font> I say average because there are so many other factors that are mixed in that have nothing to do with the cartridge like barrel condition and air temperature. And these things can and do have an effect on velocity variations that make it hard to seperate what is what.

If you were to take ten 6ppc's and ten 204 Rugers and run 5 different loads through each of them and take the average standard deviation, the 6 ppc would still be in the teens or single digits most likely and the 204 could be as high as 50 fps variation! Why? You are comparing a tall, skinny case to a short fat one.

Now look at the expansion ratios of the two in 22" barrels. YOu will see that the 6ppc has a better ratio which means it is more efficient. <font color="blue"> So, the case design gives lower standard deviations on average and makes better use of the available powder! </font>

You might say, "yeah GG but the 6ppc burns more powder than the 204, so how can it be more efficient?" Well, if you look at the weights of the bullets being used and the lengths of the barrels needed to attain sufficient velocity, you will see the ppc only uses about 5 more grains of powder to push a 28 grain heavier bullet from a shorter optimum barrel length.

So, to answer your question 4kie,
<font color="red">CASE DESIGN IS EVERYTHING!!!!!!!! </font>

If you want to read more on this great subject, ML McPherson is currently writing a great three part discourse on this very thing in the VH magazine. His results show that unburned powder actually SLOWS the gas expansion and reduces velocity comparitively! It's a very good read. [img]/ubbthreads/images/graemlins/wink.gif[/img]
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  #6  
Old 12-02-2005, 01:17 AM
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Re: A question for the ballistic scientists, err, Wildcatters.

Great stuff everyone. I have been reloading for years and never actually took the time to research what was meant by case efficiency. This helps ALOT when trying to answer the question of why wildcatters will design a round that has the same, or very similar, external ballistics to something already in existance. It makes absolute sence that a person would want a cartridge that makes better use of the powder put into it or one that lends it's self to more consistant velocities or both if possible.

It also opens a whole new realm of thought as to what the cartridge designers like Weatherby and Gibbs or Ackley and Lazzeroni were trying to accomplish. It helps to answer questions on why so many rounds are based on other rounds like the 7mm-08 and .243 or the .25-06 and the .338-06 or the .35 Whelen.

The physical attributes of cases and case forming as well as the way cases behave as part of a machine have always been the obvious answer to this question but now I see that the original designers may have (probably were) also after something more than simply necking up or down something or reducing bolt thrust and feeding issues.

Good work guys. That is exactly what I love about this forum.

GG I will be looking for that article. Thanks for the notice.
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  #7  
Old 12-02-2005, 06:59 AM
 
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Re: A question for the ballistic scientists, err, Wildcatters.

4kedhorn

Well...what this does is really simple; the theory of a MORE EFFICIENT CASE makes folks BUY MORE RIFLES....which is GOOD for the folks at RemChester!! With the quality of powders that we have today....much can be done toward making a case....MORE EFFICIENT!! [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]
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