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stevewhr
Reply with quote  #1 
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
sleepygator
Reply with quote  #2 
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
Reply with quote  #3 
Well I’m no expert but this is what I think. After running the numbers and reviewing a couple of reloading books .280 produces about 50fps more velocity than the .284 (again all things being equal).

What I think you are seeing is the difference between .280 Remington factory loaded hunting rounds being fired from factory hunting rifles with 24 inch barrels verse .284 Winchester hand loaded target rounds being fired from custom target rifles with 26 plus inch target barrels. Hunting rounds are most likely able to fit in a magazine as where target rounds typically will not. Target rounds typical have the bullets seated out farther to engage the lands, therefore giving more volume to the case. Then if you add the longer target barrel you will now have a .284 cartridge that will surpass the typical larger .280 cartridge.
stevewhr
Reply with quote  #4 
I suspect you're onto something, however the earlier post suggest a differential of inherent capabilities of short fat "guys". I'd be interested to know just how valuable short & fat really is. To be sure the 284 gets into trouble with cartridge overall length. So I'm guessing, perhaps some of both. Makes me ponder giving my 280 an improved chamber, thus taking advantage of what there is to gain by increased volume and better case design. Has anyone tried R17 in the 280? That alone could make a improvement...My apologizes, but I couldn't make heads or tails of the info about the military "grenade" or whatever. Sorry.
glo
Reply with quote  #5 
I am not a proponent of short & fat. However, I do believe a 40 degree should will tend to less the case streching phonominum (pardon any misspelings I have no spell check at this time). Less case strching will cuase less velocity loss.

In my exsperance the best accuracy comes first from a good barrel. Second from a good bullet and third from turning case necks. Velocity is a matter of case capacity, powder and barrel lenght for any given bullet weight.
sleepygator
Reply with quote  #6 
Sorry for any ambiguity and to explain what I intended:

The ideal chamber for an explosion or rapid burning is a sphere, ignition initiated in the center. We cannot practically achieve that but can compromise to obtain the best chamber design possible.

The most complete, consistent combustion occurs at maximum pressure. If you are pushing powder down the bore, pressure is rapidly dropping and combustion is less predictable. The more burning that can occur within the case, the better. Many medium caliber (~15-75mm) cartridges employ flash tubes that ignite the powder closer to the projectile and burning toward the rear. Flash tubes significantly reduce the amount of powder burned in the bore.

Long, narrow, shallow-shouldered cartridges are not as consistent. Cartridges like the 6mm BR and 6mm PPC with small primers and flash holes, and short, fat cases, burn very efficiently and uniformly. Whether one is a proponent of this design or not is not relevant. The cartridges have recently displayed the best accuracy tend toward the SFSS (short, fat sharp shoulder) category.

Does that mean you must shoot one? Thankfully, no. Will some other cartridge anecdotally show excellent accuracy? Yes.

The QuickLOAD attachments show that the .284 provide ~60fps more velocity at the same charge weight. The .280 has ~2 grains more water capacity and can definitely hold more powder. Note that I normalized the seating depth to remove the OAL problem. Which cartridge is more efficient? The .280 can produce more velocity than the .284 but burns more powder doing it.

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