Case Velocity VS Velocity VS pressure...

[ISS]

Every time I read about some Vunderbar new boutique 6mm/257/6.5mm short cartridge that is delivering 1" groups at 1000+ yards my curiosity grows. Tony Boyer told me once at a Super Shoot that "Short and Fat is where it's at..." with regard to short range BR. He also told me if people knew the pressure most of the 6PPC's being shot there generate, they might have a stroke.

All the gentlemen I talked to at the King of Two Miles Match said bigger case capacity and better BC bullets were the answer to winning accuracy.

I grew up understanding that there are only two ways to build speed in any cartridge. They are: large capacity case, and high pressure. Case shape cannot increase velocity.

So, how are you gentlemen accomplishing these velocity increases?

ISS

 

[Fast14riot]

I think the short 'n fat is better at more consistent velocity due to the shape of the pressure wave through the case than a long skinny case. Not necessarily better at gaining high velocity. Kinda like the 6BRA being a little better than the 6Dasher at agg'ing at 1000y.

 

[old_heli_logger]

Watching this thread, great topic thanks!
My 6ppc is a rat gun, so I don't push it too hard.
Good luck!

 

[6.5 forever]

Good topic. Watching for input!

 

[std7mag]

In my thinking, those shooting ELR/ULR are wanting a large bullet caliber to get the weight of the bullet up to help with wind deflection.
The heavier the bullet, the less deflection.
Obviously BC helps with this in the actual form of the bullet.
Getting higher BC, usually dictates that the bullet is longer, thus increasing weight.
The larger case capacity is to get the bullet to the desired velocity to reach the distance.

Something heard alot of in the USA is the ELR/ULR shooters liking large caliber bullets to "help see the splash" where the bullet lands.
I say that, that entirely depends on the environment in which you are shooting.

 

[6.5 forever]

My 6.5 bullet does not make alot of dust at 1 mile...for sure. If I was shooting that distance continually, a bigger bullet would be better.

 

[JEFFPPC]

I think the short 'n fat is better at more consistent velocity due to the shape of the pressure wave through the case than a long skinny case. Not necessarily better at gaining high velocity. Kinda like the 6BRA being a little better than the 6Dasher at agg'ing at 1000y.

I agree and will add I doubt the guys strictly chasing velocity are also chasing the winners.

 

[357Mag]

Every time I read about some Vunderbar new boutique 6mm/257/6.5mm short cartridge that is delivering 1" groups at 1000+ yards my curiosity grows. Tony Boyer told me once at a Super Shoot that "Short and Fat is where it's at..." with regard to short range BR. He also told me if people knew the pressure most of the 6PPC's being shot there generate, they might have a stroke.

All the gentlemen I talked to at the King of Two Miles Match said bigger case capacity and better BC bullets were the answer to winning accuracy.

I grew up understanding that there are only two ways to build speed in any cartridge. They are: large capacity case, and high pressure. Case shape cannot increase velocity.

So, how are you gentlemen accomplishing these velocity increases?

ISS

ISS -

Howdy !

Renowned ballistician Homer Powley mentioned this this in his “ Powley Computer “
instructions manual:

“ For conventional types of guns there are only three ways by which higher than normal velocity may be obtained.

1. Increase pressure. This is usually unsatisfactory because a 10% increase in pressure increases velocity by only half such a percentage, namely 5%.

2. For a given bullet weight, increase total volumne of the gun by increasing the
Expansion Ratio. You rapidly run into limitations because this means that for a given cartridge you have to increase barrel length. Sometimes the gun becomes unwieldy or too heavy.

3. For a given bullet weight, increasing the total volumne of the gun by decreasing the expansion ratio. This is the method most usually used successfully. For a given gun length the chamber is increased in size. This causes more powder to be burned with more energy and increased velocity. However, the lower limit for Expansion Ratio is usually about 5. “

The 2 Mile shooters were telling you what level of ballistic performance has proven necessary to win @ that distance.


With regards,
357Mag

 

[DirtySteve]

This has been posted before but it is worth posting again. From an actual rocket scientist.

https://web.archive.org/web/2022070...istics.com/efficiency-why-case-shape-matters/

It comes down to efficiency. Short and fat get ignited better with out pushing a plug of powder into the barrel to be burnt there. Meaning less heat generated in the barrel/less wear. There are other articles on the link above worth reading.. My reply is a very condensed version of why..

 

[Capt. Oblivious]

Case design/shape (Ackley Improved vs a case with lots of taper) can hide pressure and indirectly increased speed

 

[cmysix]

try and get any velocity out of a straight walled cartridge.

 

[sandwarrior]

Usually it's because of the effiency of the cartridge and... the length of the barrel. I have found that AI cases may save brass, but not if you load too hot. Same with barrels. you don't save them if you are loading high pressures. Effeiciency must also take into mind that using too much is inefficient.
The length of barrel allows pressure that is generated to be used. A shorter barrel may not be able to use the pressures generated. A long barrel of course means less wieldy, which is fine for F-Class but, not getting in and out of mobile vehicles. Too long is also less wieldy hunting in the field.
Cartridge shape also affects efficiency. The shoulder angle is spit between improving compression of the load during firing and allowing flow. Depending on the bore size you find more velocity by increasing the shoulder or in smaller cases you might find velocity decreasing with too much shoulder angle increase.