Col. Batguano
75 Cal.
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There are a few advantages to progressive rifling that haven't been discussed. What I'll discuss here is heat;
When we look at the event of the main charge igniting, and sending the projectile on it's way, there is a tremendous acceleration / g- forces of the projectile involved. With softer mediums like lead, that acceleration tends to compress it, and the pressure squeezes and obdurates it so it can fill the grooves of the rifling. (Round balls coming out of a barrel are always somewhat flattened, and show some rifling marks.) That swaging process generates a lot of heat through friction. (Anecdote; The Earth's core is around 8000 degrees. That heat is largely due to the friction generated by the internal liquid materials due to the daily effects of a gravitational directional differential from the moon. There are currents within the Earth's interior.) Similarly, the projectile is being accelerated from a spin rate of zero to many thousands of rpm's. The rifling essentially "grabs" the bullet and imparts a spin to it. Again, that acceleration and friction (with the barrel) necessary to spin it creates heat. In a gain twist barrel, less heat is generated in the initial slower twist chamber area, but is more spread out over the entire barrel length as the bullet's spin rate is increased, resulting in less throat erosion. Hot metal is less resistant to erosion than cold metal. The Italians used this phenomenon when they made their Carcano rifles, in an effort to help keep barrel heat down over a sustained firing string, and extend barrel life.
Note; Have you ever looked at the base of a swaged bullet after it's been shot? It's usually all dimpled to mirror the shape of the unburned contacting powder granules. That's a lot of pressure to dent those in there. With round balls, some times the cloth weave pattern is slightly imprinted too.
The second item is that, with smaller charges that don't obdurate the bullet as much is that with a slower twist the ball is less likely to "jump the rifling" (with slower twist rifling and not fully engage it.
The progressive depth thing wouldn't really do that much to mitigate heat build-up. In fact, because of the continual swaging of the bullet, it would generate some more along the way rather than less. But, if the bullet didn't fully swage in to the grooves earlier, the shallower nature of them toward the muzzle would be more sure to make sure the bullet got the full spin. The gas blow-by would have already happened closer to the breech.
Thermal imaging tells us that surface temperatures of modern gun bullets is between 100C and 300C (which is hot enough to sterilize them). Most of that temperature is generated through barrel bore friction.
When we look at the event of the main charge igniting, and sending the projectile on it's way, there is a tremendous acceleration / g- forces of the projectile involved. With softer mediums like lead, that acceleration tends to compress it, and the pressure squeezes and obdurates it so it can fill the grooves of the rifling. (Round balls coming out of a barrel are always somewhat flattened, and show some rifling marks.) That swaging process generates a lot of heat through friction. (Anecdote; The Earth's core is around 8000 degrees. That heat is largely due to the friction generated by the internal liquid materials due to the daily effects of a gravitational directional differential from the moon. There are currents within the Earth's interior.) Similarly, the projectile is being accelerated from a spin rate of zero to many thousands of rpm's. The rifling essentially "grabs" the bullet and imparts a spin to it. Again, that acceleration and friction (with the barrel) necessary to spin it creates heat. In a gain twist barrel, less heat is generated in the initial slower twist chamber area, but is more spread out over the entire barrel length as the bullet's spin rate is increased, resulting in less throat erosion. Hot metal is less resistant to erosion than cold metal. The Italians used this phenomenon when they made their Carcano rifles, in an effort to help keep barrel heat down over a sustained firing string, and extend barrel life.
Note; Have you ever looked at the base of a swaged bullet after it's been shot? It's usually all dimpled to mirror the shape of the unburned contacting powder granules. That's a lot of pressure to dent those in there. With round balls, some times the cloth weave pattern is slightly imprinted too.
The second item is that, with smaller charges that don't obdurate the bullet as much is that with a slower twist the ball is less likely to "jump the rifling" (with slower twist rifling and not fully engage it.
The progressive depth thing wouldn't really do that much to mitigate heat build-up. In fact, because of the continual swaging of the bullet, it would generate some more along the way rather than less. But, if the bullet didn't fully swage in to the grooves earlier, the shallower nature of them toward the muzzle would be more sure to make sure the bullet got the full spin. The gas blow-by would have already happened closer to the breech.
Thermal imaging tells us that surface temperatures of modern gun bullets is between 100C and 300C (which is hot enough to sterilize them). Most of that temperature is generated through barrel bore friction.
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