OlongJohnson
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Topic that's been hashed over countless times. But ideas here that I haven't seen anywhere else.
Standard link: BBTI - Ballistics by the Inch :: .357 Mag Results
The upper table was made by progressively cutting a Contender barrel to shorter lengths. My understanding is that the length in the table is the overall length, including the chamber. Thus, a key detail in interpreting it would be to account for the case and freebore length - subtract 1.4 inches from the lengths in the leftmost column to get a more realistic understanding of effective barrel length.
The lower table was made by firing unmodified firearms. A link to this page is posted in literally every discussion I've seen on the topic, and someone usually says that data is "suspect" or something like that. I'm not so sure.
I notice that of the revolvers fired, there's a significant increase from the 3" barrel to the 4" barrel, but realistically, the differences in velocities for all the barrels from 4" to 6" in real revolvers are probably within the range of shot to shot variation of non-match-tuned ammo. One might at first think the 6" Colt was long enough to be slowing down the bullet, but the 5 7/8 Korth velocities make it seem more likely that the Colt is just slow. The two firearms that achieve significantly greater velocity are carbines without cylinder gaps.
This suggests the hypothesis of a functional velocity plateau beginning somewhere around 4 inches for a .357 Magnum revolver. It's not terribly surprising. As the bullet moves down the barrel, pressure in a "sealed" barrel rises and then drops. In a revolver, once the bullet passes the cylinder gap, gas is being bled out of the cylinder gap at a very high rate, considering the pressures involved, so the drop in pressure will be much faster. All you need for a velocity plateau is for the pressure loss at the cylinder gap to be sufficient that bullet velocity is only maintained against friction from the barrel.
Why would this not be observed with .44 Magnum? Well, the area of the cylinder gap is proportional to the bore, while the volume of the chamber is proportional to the square of the bore. So the size of the "open door" that gases flow out of is proportionately ~20 percent greater for the .357, assuming the same width of cylinder gap. At the same time, to use rough numbers, a typical .44 Magnum powder charge is about two-thirds larger than a typical .357 Magnum powder charge, so there's a lot more volume of gas to bleed down. As well, the area of the bullet that the gas is pressing against is proportional to the square of the bore, while the bore circumference, somewhat correlated to the area to generate sliding friction, is proportional to the bore diameter.
You'd expect that a .44 would run into a velocity plateau eventually, but you'd expect it to be with a longer bore. And in fact, we don't see it with any common pistol-length barrel. Although, the 12" 629 at BBTI - Ballistics by the Inch :: .44 Mag Results does seem to be getting into diminishing returns, and one could reasonably hypothesize that it is past the start of a plateau. Would be nice if there was an 8 3/8 in there.
Both cartridges continue building velocity to 20" and beyond when used in carbines that don't have a cylinder gap.
Which isn't to suggest that there's no benefit to a .357 revolver barrel longer than 4 inches. Sight radius, balance, rotational moment of inertia helping to manage sighting oscillation and muzzle flip under recoil are all shooter benefits. As well, if pressure is rapidly bleeding out of the space behind the bullet, it's possible that the pressure when the bullet leaves the barrel will be lower, providing slight reduction in sound pressure from the muzzle blast. Powder burn is liable to be more complete, reducing flash (and noise) in low-light conditions.
But there may be a combination of circumstances at play that means going to a longer barrel than 4" in a .357 Mag won't consistently get higher velocities, and thus that no additional defensive "firepower" will be obtained with a larger pistol.
If someone planned to rebarrel an 8 3/8 686 at some point, it would be a pretty cool to test this hypothesis with precisely-controlled hand loaded ammo, eliminating all variables except barrel length.
Thoughts?
Standard link: BBTI - Ballistics by the Inch :: .357 Mag Results
The upper table was made by progressively cutting a Contender barrel to shorter lengths. My understanding is that the length in the table is the overall length, including the chamber. Thus, a key detail in interpreting it would be to account for the case and freebore length - subtract 1.4 inches from the lengths in the leftmost column to get a more realistic understanding of effective barrel length.
The lower table was made by firing unmodified firearms. A link to this page is posted in literally every discussion I've seen on the topic, and someone usually says that data is "suspect" or something like that. I'm not so sure.
I notice that of the revolvers fired, there's a significant increase from the 3" barrel to the 4" barrel, but realistically, the differences in velocities for all the barrels from 4" to 6" in real revolvers are probably within the range of shot to shot variation of non-match-tuned ammo. One might at first think the 6" Colt was long enough to be slowing down the bullet, but the 5 7/8 Korth velocities make it seem more likely that the Colt is just slow. The two firearms that achieve significantly greater velocity are carbines without cylinder gaps.
This suggests the hypothesis of a functional velocity plateau beginning somewhere around 4 inches for a .357 Magnum revolver. It's not terribly surprising. As the bullet moves down the barrel, pressure in a "sealed" barrel rises and then drops. In a revolver, once the bullet passes the cylinder gap, gas is being bled out of the cylinder gap at a very high rate, considering the pressures involved, so the drop in pressure will be much faster. All you need for a velocity plateau is for the pressure loss at the cylinder gap to be sufficient that bullet velocity is only maintained against friction from the barrel.
Why would this not be observed with .44 Magnum? Well, the area of the cylinder gap is proportional to the bore, while the volume of the chamber is proportional to the square of the bore. So the size of the "open door" that gases flow out of is proportionately ~20 percent greater for the .357, assuming the same width of cylinder gap. At the same time, to use rough numbers, a typical .44 Magnum powder charge is about two-thirds larger than a typical .357 Magnum powder charge, so there's a lot more volume of gas to bleed down. As well, the area of the bullet that the gas is pressing against is proportional to the square of the bore, while the bore circumference, somewhat correlated to the area to generate sliding friction, is proportional to the bore diameter.
You'd expect that a .44 would run into a velocity plateau eventually, but you'd expect it to be with a longer bore. And in fact, we don't see it with any common pistol-length barrel. Although, the 12" 629 at BBTI - Ballistics by the Inch :: .44 Mag Results does seem to be getting into diminishing returns, and one could reasonably hypothesize that it is past the start of a plateau. Would be nice if there was an 8 3/8 in there.
Both cartridges continue building velocity to 20" and beyond when used in carbines that don't have a cylinder gap.
Which isn't to suggest that there's no benefit to a .357 revolver barrel longer than 4 inches. Sight radius, balance, rotational moment of inertia helping to manage sighting oscillation and muzzle flip under recoil are all shooter benefits. As well, if pressure is rapidly bleeding out of the space behind the bullet, it's possible that the pressure when the bullet leaves the barrel will be lower, providing slight reduction in sound pressure from the muzzle blast. Powder burn is liable to be more complete, reducing flash (and noise) in low-light conditions.
But there may be a combination of circumstances at play that means going to a longer barrel than 4" in a .357 Mag won't consistently get higher velocities, and thus that no additional defensive "firepower" will be obtained with a larger pistol.
If someone planned to rebarrel an 8 3/8 686 at some point, it would be a pretty cool to test this hypothesis with precisely-controlled hand loaded ammo, eliminating all variables except barrel length.
Thoughts?
