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What makes a cartridge efficient

calgarycanada

Here we talk about external ballistics mostly but I have been baffled by the cartridge efficiency, that is amount of powder burnt vs velocity generated using same bullet.
Not discussing accuracy here.

What makes a cartridge more efficient then the other?
Is it just the volume or is it the cylinder diameter(whatever the technical term is) vs length of case ratio? Does shoulder angle play a significant role as well?
Or is it that certain chamber volume is more adequate for certain powders and there is no other combo of powder & bigger volume case?

I started thinking about this when I was comparing 6mm Dasher with 6x47 lapua. At similar pressures dasher can push same bullet at almost same velocity. I know 6x47 is rated for higher pressure so can be pushed to higher velocities but if pressures as kept similar, say 61000psi, velocities are similar as well.
I ran numbers through Quickload and tried couple of different Hogdon powders, but the results are same. At similar pressure 6x47 can push 105s slightly faster (not by much)but it uses 10 grains more powder compared to dasher.

Why is that? Why ~15% more powder only translates in to < 2% velocity increase?

What if we keep same dimensional ratios (length vs diameter), same taper, same shoulder angle etc can we make a bigger cartridge that is as efficient (powder vs velocity) as Dasher?? Obviously if it was possible cartridge manufacturers would have been doing it already, just wanna know "why" part of it.
 
calgarycanada said:
I was running 6mm 105 hybrids. Shank seating depth (length of bullet in neck)0.150" in both cases.
Click to expand...
I had 6- 47 L yes it was a very little faster . But never got it to shoot . Chambered to A Dasher . Now I have 7.

6 ppc is lite bullet case dasher or Brx is heavy 6.5 wssm is 140 GR they all are near perfect as can be had for powder used and speed. Another case that looks good is a 6 -45 . Larry
 
Here is some food for thought.. A .22 LR sends a 40 gr bullet out at 1200 f.p.s. with about 4grs of powder. A .223 sends a 40gr bullet out at 3600f.p.s. with about 26-27 grs of powder. You are getting a 300 percent increase in velocity BUT it takes 600-700 percent powder increase for a 300 percent increase in velocity. Take a 6mmBR (with a 24" barrel) that shoots a 105gr bullet with about 30.0grs of powder, at "about" 2700f.p.s. Now look at the .243Win. that shoots the same bullet, at 2800f.p.s. It takes about 48.0grs of H4831SC to get there. It took ALMOST 30 percent MORE powder to move 100 f.p.s. faster.

[Boss: I am getting 2910 fps with 105s in 6BR 27" 3-groove, 30.3 gr Varget. That is certainly faster than average, but it's doable in some barrels.]

What we have here is "The Law of Diminishing Returns" showing up in a BIG way. The key is to design a case that burns LESS powder and still get near top velocities. The "short / fat / steep shoulder" designs appear to convey that concept. EXAMPLE: As stated before, a 24" barrel, out of a 6mmBR will deliver 2700 f.p.s. from a 6mmBR. Increase the same case "about" 10 percent, make a 40 degree shoulder and get 150f.p.s. faster! You just "created" a 6 Dasher. How can that Dasher get 150f.p.s more with a 10 pct increase and a .243 get 100 f.p.s. increase with over a 50 pct increase in powder? By the way, to get these "numbers" I am using the Hodgdon loading manual. The only thing I can see is CASE DESIGN! But I am not a ballistician... So, if we really want to get "good info" instead of a guess, Bryan Litz might be enlisted to help answer this question..
 
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ShootDots, that's exactly what puzzles me. I understand straight walled cases are more efficient then bottle necked , so 22 LR vs 223 I kind of understand but you are right, 6mm br/dasher vs 243/243AI doesn't add up; even though AI version has similar shoulder angle it "wastes" lot more powder.
So question for Bryan is what is the cause of this "point of diminishing returns " in internal ballistics and can it be fixed/reduced somehow by increasing dimensions of a case but keeping same dimensional ratios as BR/Dasher, ie shorter/fatter?
 
Cartridge efficiency is a computed percentage of the amount of transferred energy to the bullet, from the amount of powder (charge) and that powders energy level.
Here is the formula I use to compute cartridge efficiency:

((Bullet Weight * .5) * (MV ^ 2) / ((Charge * BTUjoules) / 98000)))
​
Which is computed from bullet weight, muzzle velocity, charge, and powder energy.
Donovan
 
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dmoran said:
Cartridge efficiency is a computed percentage of the amount of transferred energy to the bullet, from the amount of powder (charge) and that powders energy level.
Here is the formula I use to compute cartridge efficiency:

((Bullet Weight * .5) * (MV ^ 2) / ((Charge * BTUjoules) / 98000)))
​
Which is computed from bullet weight, muzzle velocity, charge, and powder energy.
Donovan
Click to expand...
Hey Donovan! My math skills are about as good as my long-range shooting! LOL!! I can barely add and subtract! I have no doubt of that formula, I would just never be able to compute anything like that! Far beyond my capacity!
 
Donovan,
I'm going to have to copy your comment and post it above my reloading bench so I can remind myself of what I'm doing wrong. And you are just the guy I wished had sat next to me in a college Algebra class I took in the mid 60's. In fact, I was so lost in what was being taught, I quit going. But your formula makes alot more sense to me today and I can even relate after all these years. Now perhaps (like ShootDots says) I can improve on my long range shooting. Thx!

Alex
 
If you compare a larger case using a slower burning powder to a smaller case with a faster burning powder, the latter will appear more "efficient". Will it not?

What controls and/or constants need to be in place for an efficiency comparison to have any viability?
Chamber pressure?
Velocity?
Barrel life?

If a smaller case can run a higher pressure charge, thanks to a thicker web of brass (via small primer pocket), that goes a long way in establishing the perceived "efficiency" of said case. Especially if all your looking for is charge volume - velocity ratio...
 
Here is the formula I use to compute cartridge efficiency:

((Bullet Weight * .5) * (MV ^ 2) / ((Charge * BTUjoules) / 98000)))
​
Donovan

Where do you get the heat of combustion data? is the data given in BTU's or Joules? and that doesn't create the need for a different conversion constant.
 
In a nutshell...small cases with large bores allow for using faster powders that are more efficient. We must work within certain pressure limitations. I like to look at chamber volume to include to portion of the bore to where peak pressures occur. Larger bores create a larger expansion area and a larger driving surface on the bullet, creating more thrust or energy, while smoothing out the pressure curve.

Bottom line, small case, large bore, fast powder= efficient cartridge design. You can search this site for a cartridge overbore comparison chart. I don't think it's a coincidence that some of the most accurate cartridges ever designed are also some of the least overbore designs. The area under the curve in a graph of chamber pressures is indicative of great efficiency and I believe, inherent accuracy potential. Of course, we have to have good bullets, barrels and a host of other things before potential accuracy can be fully realized. The cartridge that I have found to have the most area under the curve just happens to be the same cartridge design that holds the world record for the smallest 5 shot 100 yard group ever fired in competition. Again, I don't think this is by coincidence. That cartridge is a 30 caliber Grendel case. I call my version of it a 30 Major. When I developed it, I used a Pressure Trace system to make comparative graphs of the pressure curves against several different cartridges. It is still the most efficient cartridge that I have ever tested. Others may define efficiency differently, but there is a relevance to my method, particularly when used comparatively.
 
HPMike800 -
There is several resources to "Heat of Explosion" data for powders.
The powder manufactures of course, but also QuickLoad is a common source (given in kilo/joule).
The conversion to BTU that I use is a .4297 multiplier ( Example: kJ/kg x .4297 = BTU / lb.)
Donovan
 
gunsandgunsmithing said:
In a nutshell...small cases with large bores allow for using faster powders that are more efficient. We must work within certain pressure limitations. I like to look at chamber volume to include to portion of the bore to where peak pressures occur. Larger bores create a larger expansion area and a larger driving surface on the bullet, creating more thrust or energy, while smoothing out the pressure curve.

Bottom line, small case, large bore, fast powder= efficient cartridge design. You can search this site for a cartridge overbore comparison chart. I don't think it's a coincidence that some of the most accurate cartridges ever designed are also some of the least overbore designs. The area under the curve in a graph of chamber pressures is indicative of great efficiency and I believe, inherent accuracy potential. Of course, we have to have good bullets, barrels and a host of other things before potential accuracy can be fully realized. The cartridge that I have found to have the most area under the curve just happens to be the same cartridge design that holds the world record for the smallest 5 shot 100 yard group ever fired in competition. Again, I don't think this is by coincidence. That cartridge is a 30 caliber Grendel case. I call my version of it a 30 Major. When I developed it, I used a Pressure Trace system to make comparative graphs of the pressure curves against several different cartridges. It is still the most efficient cartridge that I have ever tested. Others may define efficiency differently, but there is a relevance to my method, particularly when used comparatively.
Click to expand...
One other thing the shoulder design on the brass can change the efficiently Bi Smally made a hemispherical chamber. Larry
 
GSXR said:
The cartridge that develops the highest energy with the fewest grains of powder is the more efficient cartridge.
Click to expand...

I agree.

But...now consider this:

Cartridge "A" burns 50.0 gr of powder to generate 3600 fps from "X" bullet in a 28" barrel.

Cartridge "B" burns 46.0 grains of powder to generate 3700 fps from the same "X" bullet, same barrel.

Which is more efficient? A or B?

One would think "B", by default of the above logic.
However....

What if "A" and "B" are actually the SAME CARTRIDGE?
What if "B" is simply running a faster burning powder to generate more speed, from less volume?

This is an entirely possible & plausible scenario...

To the point:

See where blanket statements don't tell the whole story on cartridge "efficency"?

See how controls/constants need to be in place, in order to draw a viable conclusion between cartridges???

Just sayin', don't be so quick to label a cartridge as more "efficient" based on volume-velocity ratio alone. Too easy to cherry pick powders & pressure curves that suit one over another...
 
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