Cartridge Efficiency

[dak0ta]

Can somebody suggest some good reading on how to understand what makes an efficient cartridge based on case design, powder burn rate, projectile weight and shape, and bullet diameter?

I've read that the .308, .30-06, 6.5x55, 270 Win, 8mm Mauser are all efficient cartridges if shooting the heavier bullets for caliber. For example, the 8mm Mauser is more efficient shooting a 200gr bullet compared to the .30-06. Is this because the .323 caliber's slightly increased bore diameter over .308 cal allows for more surface area for the expanding gas pressure to push the heavy for caliber bullet down the barrel, hence providing more velocity for a relatively equivalent powder charge in the 30-06 and 8x57 cases? But a 30-06 can shoot the 165-180gr bullet more efficiently vs the 8x57 because the lighter bullet weight is more efficient in the smaller caliber?

 

[Varminterror]

Pick up Ackley’s book. Then follow some of the WSM data beyond that. Lots of info out there in those particular pathways.

 

[jmr40]

There is bullet efficiency, and cartridge efficiency.To explain bullet efficiency you have to look at the effects of Ballistic coefficient (BC).

Using Nosler Accubonds a 200 gr 30 caliber bullet has a BC of .589, a 200 gr 8mm bullet is .450. Assuming you could get 50 fps more speed out of the 8mm it has an advantage at the muzzle of about 125 ft lbs energy. But at only about 75 yards the 30-06 bullet is moving faster and hitting harder because the bullets are more aerodynamic. And the farther you go down range the greater the advantage 30-06 has over 8X57. At 300 yards the 200 gr 30-06 is significantly more powerful than the 8X57.

Any time you're shooting bullets of similar weights and powder charges from 2 different calibers the larger caliber will always leave the muzzle faster. But assuming you're using the same bullet, the smaller caliber will always catch up at some point down range, hit harder, and shoot flatter.

Then there is cartridge efficiency. Generally speaking short fat cases are more efficient than long skinny ones. A 308 shoots the same bullet weights as 30-06. I can shoot 155 gr target loads out of my 30-06 @ 2950 fps, but will need 60 gr of powder to get there. The same bullets from my 308 will get 2800 fps, but with only 46 gr of powder. My 308 gives up about 5% speed, but with 25% less powder and about 30% less recoil. While 30-06 is faster, 308 is more efficient.

 

[redneck2]

John Barsness had a really good article years ago in Handloader Magazine. He probably knows as much about real, live ballistics as anyone I’ve found.

His whole premise was, there is no such thing as an “efficient” cartridge.

OK, here it comes!

“BUT....BUT....a _________ (WSSM, Ackley, etc.) GETS MORE SPEED THAN A _______!!!!!! THATS PROOF!

Uhhhhh....no. Something about laws of physics that can’t be changed. The “efficient” cartridges run at higher pressures. Use the same case with more powder equals more pressure. Use a faster powder in the same case, more pressure.

Guys get all wrapped around the axle about 1/2 a degree of change to the shoulder angle, a 11 degree case angle versus 12 degree.

A given bullet at a given speed is going to perform about the same no matter the head stamp, shoulder angle, or case capacity. Higher pressure in a particular chambering gives a higher velocity than lower pressure. A 10% increase in powder capacity gives a 2 1/2% increase in velocity, at the cost of more muzzle blast and lower barrel life.

Someone will be along shortly to nit pick about. .005% change. Whatever. Get off the keyboard, get behind the rifle, and pull the trigger. Then tell me it’s a big deal. If you like what you you like, have at it.

In the immortal words of the Captain in Josie Wales, “don’t piss down my back and tell me it’s raining”

 

[mcb]

How deep down that rabbit hole do you want to go? If you're willing to take the red pill and a background in dynamics and thermodynamics wouldn't hurt then I would suggest two.

Ballistics: Theory and Design of Guns and Ammunition, by Donald E. Carlucci and Sidney S. Jacobson. They are on the third edition of this college text book but if you can find either of the two previous versions it would suffice nicely. Its expensive so finding an older version used would be a good way to get the information cheaper. I worked briefly and indirectly with Carlucci in a previous job. About a 1/3 of the book is dedicated to internal ballistics. It also covers exterior and terminal ballistics and design elements.

The second source is even more useful. Pick up a copy of Quickload software. Again not cheap but very useful. This is an internal ballistics simulator that uses a model derived from the principles you will learn in the previous book. There is also a detailed explanation of their model and the physics behind it in the documentation that comes on the CD with the software. It is not a thorough as the previous text but it is a pretty good explanation of what happens in internal ballistics.

The second source is probably the better investment as it will be useful after you sate your questions of efficiency.

 

[dak0ta]

Thanks fellas, I've got the time to nerd out on this stuff at the moment.

 

[Llama Bob]

John Barsness had a really good article years ago in Handloader Magazine. He probably knows as much about real, live ballistics as anyone I’ve found.

Well, you've provided today's gut laugh I'll stipulate he THINKS he knows more than anyone else.

The internal efficiency of guns, cartridges and their loadings is a well understood scientific topic. It's measured by taking the chemical energy present in the powder charge (measured by burning in a calorimeter) and comparing it to the kinetic energy of the projectile leaving the barrel. Since energy is conserved, all the chemical energy in the powder ends up as either kinetic energy in the bullet, waste kinetic energy in the action (for auto actions) and escaping gas, or heat. Efficiency is simply bullet kinetic energy divided by total energy.

Now, if you want to know what makes a gun system efficient, the following factors apply:

• pressure - it's possible to achieve higher efficiency at higher operating pressures
• powder burn rate - faster powders tend to be more efficient BUT slower powders let you use more powder, which gives higher velocity but in a less efficient way.
• barrel length - the longer the barrel, the more energy is converted into bullet KE and the less into heat and waste KE
• bore diameter - if you keep the case size and powder burn curve the same, larger bores are more efficient
• friction - the less bore friction there is, the more efficient the system is. For many designs including most rifles, bore friction is fairly negligible
• Sebert's factor - the energy from the powder accelerates both the bullet AND the remaining unburnt powder down the barrel. The energy imparted to the powder is wasted. Cartridges with large case diameters relative to bore diameter and sharp shoulders have better Sebert's factors and keep the powder put better, wasting less energy. Straight walls are the worst in this regard. Sebert's factor is most important for artillery (both land and naval) but it matters for rifles too. When someone says a cartridge is "efficient" they're usually talking about a low (good) Sebert's factor. This is why the Ackley cartridges and modern cartridges of similar design are often referred to as "efficient".
• Burning powder inside the barrel - powder burned outside is all waste heat.

 

[Varminterror]

Anything which uses anything measurable to produce anything measurable can easily be measured for efficiency. Pretty simple - anything which uses less to make more than another is “more efficient,” and anything which uses more to make less is “less efficient.”

On the most simplistic basis, a person can compare two cartridges by considering Potential Energy input and the resulting Kinetic Energy is an exceptionally simple means of calculating cartridge efficiency. Simple proxies are used for these inputs, rather than considering actual Potential Energy, we use powder charge weight, and for kinetic energy, we can either calculate KE, or simply use the resultant velocity - assuming bullet weight is held constant. In such an analysis, it’s foolish to pretend “efficiency” doesn’t exist. It’s very obvious that the same powder can be used in similar but different cartridges to produce significantly different performance efficiencies in terms of energy per charge weight.

 

[Nature Boy]

.308

 

[PapaG]

First, define what you mean by efficiency. That is the real starting point for discussion.

 

[Weflyfast]

John Barsness had a really good article years ago in Handloader Magazine. He probably knows as much about real, live ballistics as anyone I’ve found.

His whole premise was, there is no such thing as an “efficient” cartridge.

OK, here it comes!

“BUT....BUT....a _________ (WSSM, Ackley, etc.) GETS MORE SPEED THAN A _______!!!!!! THATS PROOF!

Uhhhhh....no. Something about laws of physics that can’t be changed. The “efficient” cartridges run at higher pressures. Use the same case with more powder equals more pressure. Use a faster powder in the same case, more pressure.

Guys get all wrapped around the axle about 1/2 a degree of change to the shoulder angle, a 11 degree case angle versus 12 degree.

A given bullet at a given speed is going to perform exactly the same no matter the head stamp, shoulder angle, or case capacity. Higher pressure in a particular chambering gives a higher velocity than lower pressure. A 10% increase in powder capacity gives a 2 1/2% increase in velocity, at the cost of more muzzle blast and lower barrel life.

Someone will be along shortly to nit pick about. .005% change. Whatever. Get off the keyboard, get behind the rifle, and pull the trigger. Then tell me it’s a big deal. If you like what you you like, have at it.

In the immortal words of the Captain in Josie Wales, “don’t piss down my back and tell me it’s raining”

Barsness is right on the money.....that article is worth looking up and reading....and he is the ultimate rifle loonie! .....he also has a great back of writing.....I will read his stuff even if it really isn't something I would normally be interested in.....he is relatable and a fantastic guy.

 

[redneck2]

First, define what you mean by efficiency. That is the real starting point for discussion.

By definition you’d think it’s the most velocity and/or energy generated per grain of powder.

Makes it real simple. Use an air rifle. 100% efficient! Wasn’t that easy?

 

[walterelm]

By definition you’d think it’s the most velocity and/or energy generated per grain of powder.

But this doesnt refer to the cartridge. Take a .30-06 and fill the case with titegroup .... this will give you ONE very effective shot ....

 

[mcb]

By definition you’d think it’s the most velocity and/or energy generated per grain of powder.

Makes it real simple. Use an air rifle. 100% efficient! Wasn’t that easy?

But even an air rifle is not 100% efficient. If you look at how much work/energy it took to compress the air into the reservoir versus the potential energy stored in there you will find losses have stolen some of that energy. There are losses due to mechanical friction in the pump, friction of the air moving through the pump system. Thermal losses as some of the work goes into heating the air as it is compressed and that heat is usually lost to the environment unless you shoot the air rifle immediately after compressing the gas.

Then when you compare the amount of potential energy stored in the compressed gas to what you get in kinetic energy from the projectiles you have a whole another set of losses. Again friction gets you as the bullet slides down the barrel. Some of that energy is lost to friction of the air on the barrel too. Some of that potential energy is turn into kinetic energy of the air pushing the bullet not the bullet itself. Some of that energy is lost to heat as the gases expand causing the them to cool taking some of that stored energy as pressure drop due to this temperature change.

Somewhere I have a python simulation I wrote for a air gun I used to predict the muzzle velocity of a large caliber air gun (spud gun though I was not firing spuds) and there are a fair amount of losses and I was only simulating the firing not the compression processes. It is a much much simpler problem than the simulation Quickloads has to solve.

 

[Chuck R.]

I look at it sort of like my 6mm SLR VS .243 WIN.

Exact same case, but the 6mm SLR has the shoulder pushed back to 30 degrees VS 20 of the parent case. At equal pressure the 6mm SLR uses less powder to generate the same MV as .243W. So you could say it's more efficient. Add to it the added neck length, decreased case stretch (30 degrees) and it comes across as a more efficient cartridge. When I rebarreled I just ran my .243 brass into my 6mm SLR FL bushing die, no fireforming required.

 

[denton]

There are a bunch of factors involved, but as a first cut, you can look at barrel pressure at bullet exit. When the bullet uncorks the barrel, all the remaining energy stored in the compressed gas is lost to propulsion. A 9mm cartridge in a carbine will convert more chemical energy to propulsion than the same cartridge in a handgun.

As others have correctly pointed out, efficiency is the fraction of initial energy converted to useful purpose.

 

[dak0ta]

What about WW2 military cartridges like the .303, 7.62x54r, 30-06 and 8mm Mauser that need to be designed to feed into bolt action rifles, machine guns and have reliable feeding in automatic rifles like the SVT40/G43/BAR. Is it the shoulder angle/case taper and COAL that plays a role in this? If designed to work in multiple firearms, action types, with different barrel lengths, twist rates, what specs did the engineers use to find the "general purpose cartridge" that surely had to be considered "efficient" to some degree.

 

[Varminterror]

@dak0ta - that’s versatility, not efficiency.