internal ballistics question

[stevewhr]

If we have two cartridge casings of different shape, one holding more powder than the other. Both the same caliber, all other things being equal, wouldn't the larger capacity produce more velocity? I'm thinking about the .280 and the .284, why is it that routinely, the .284 Win is reportedly giving higher velocity than the .280 Rem? The .280 Rem holds roughly 3 grains more powder. I realize that the .280 has been chambered in perhaps lesser actions, but given identical actions, wouldn't they both be limited by the same working pressure? If so, shouldn't the .280 give higher velocity potential? Insights would be most welcome........Thanks, Steve

 

[Steve Blair]

Current theory is that shorter, fat cases with sharper shoulder angles are more efficient than longer aspect ratio cases with shallow shoulder angles. It is believed that they burn more uniformly by containing the combustion instead of blowing large amounts into the barrel where burning is not as uniform.

There is probably validity to the theory. The M430A1 40mm grenade achieved amazing efficiency using a high-low pressure system that burned the propellant in a nearly spherical chamber and released the gases in a controlled manner into a low-pressure chamber that directly acted upon the projectile.,see image below)

Given that the .284 has a better aspect ratio and a sharp shoulder, it seems reasonable that it is using the powder's energy more efficiently than the 280. Ideally, barrel pressure would rapidly rise to maximum, remain constant during bullet excursion and rapidly drop to zero upon bullet exit. Since we cannot achieve that, we try to obtain the best progressivity possible by controlling burning rate and combustion characteristics.

 

[glo]

Here is why I’m not part of the crowed that has been pushing “Short and Fat” cases. I can make any long and skinny cartridge produce the same muzzle velocity as a short and fat cartridge with the same powder charge. The key is case volume after the bullet has been seated. The following data was created from “Load From A Disk” Software.

Cartridge .284 Winchester .280 Remington
Barrel Length 24 inches 24 inches
Bullet Speer 160gr SP Speer 160gr SP
Powder Reloader 19 Reloader 19
Cartridge OAL 3.265 inches 3.39 inches
Bullet Seating Depth .125 inches .370 inches
Case Capacity 62.308gr H2O 62.305gr H2O
Optimal Load Density 86% 86%
Powder Charge 53.6gr 53.6gr
Bullet to Charge Ratio .335 .335
Velocity 2603fps 2603fps
Chamber Pressure 42837cup 42839cup

What we may have over looked, when comparing different cartridge designs of similar case capacities are their designed working pressures. And no, the .284 Winchester and .280 Remington don’t have the same working chamber pressures.

The .284 Winchester has a SAMMI chamber pressure of 54.000cup/65,000psi and the .280 Remington has a SAMMI chamber pressure of 50.000cup/60,000psi. All things being equal, if you compare two different cartridges with the same bullet weight and powder charge, the cartridge that creates the higher pressure will always have a greater velocity. This is a fact and is simple physics; F=ma.

In some instances an older cartridge case design may have a greater case capacity but a low working pressure than a newer cartridge case design. If there are only a few grains difference between lower pressure-high capacity case and higher pressure-low capacity case. Then yes, the higher pressure producing cartridge will tend to create higher muzzle velocities. This is what you may have been seeing. A good, while not being perfect example, is the difference .416 Rigby and .416 Remington Mag. The .416 Rigby is lower pressure-high capacity case and the .416 Remington Mag. is higher pressure-low capacity case. Despite the case size difference the .416 Remington Mag. can equal .416 Rigby in muzzle velocity. And the reason is the higher working chamber pressure of the .416 Remington Mag.

So dose this mean the “long and skinny cartridge” of the .416 Remington Mag. is more efficient than the “long and fat cartridge” of the .416 Rigby?

 

[stevewhr]

It appears that we lost a response or two. However, thank you gator for your reply and patience. I believe that perhaps gator is right, short,fat designs are more efficient. I also believe glo is correct, in that a larger boiler room is tough to beat. So, I'm surmising, that my 280 with it's larger boiler room loaded to the same pressures as a 284, will out perform the later. I can't see any reason not to load to higher pressures, matching the 280's counterparts like, .25-06, .270 or the .30-06. Thoughts?

 

[Steve Blair]

Yes, we lost a few.

Glo is correct about total capacity. If a cartridge is loaded to the same pressure with more powder, it is likely to produce higher velocity.

Where I disagree is: If two cartridges have the same capacity and are loaded to the same pressure but one has a more favorable aspect ratio, it will likely produce higher velocity. Is the difference enough to be a deciding factor? Probably not. Can it make enough difference in burning characteristics such as extreme spread? Probably, yes. That is why these cartridge types dominate both long and short range target shooting.

 

[j_22250]

short, fat cases tend to produce not only better speeds, but also better accuracy when pushed to max pressures. wheres more traditional cartridges will tend to loose accuracy. it also almost always depends on the gun too.

 

[stevewhr]

Gator, your insights as to the current cartridge trends has helped me understand some of the reasoning behind them. I am not in a position to argue their merits. My best guess is that most mortals would not be able to recognize the differences between these newer designs and old school. I'm thinking about 1000yard records, they don't appear to have changed a "huge" amount lately. Conventional designs being at the very least competitive with the modern cases. Still, it would seem that it would behoove one to build a rifle around these newer cases if going for the "best possible". I should have been more specific about my personal reasoning behind the original question. That is, why couldn't my factory original Remington sporter run just as hard or harder as someone elses .284 in a sporter. Each rifle a carrying hunter. I understand that to some measure I'm being anal and splitting hairs. However I'm a huge fan of the .45 Colt mostly because it was/is misunderstood, and can be a hotrod/thumper or a pussy cat. So my interest in the .280 is in the same vein. Having exerted all this energy I'd like to thank you and the others who have commented on the subject. I've found it very insightful and fun to consider. Your explanation of a sphere being a most ideal combustion chamber makes a lot of sense. Thanks again. Steve.