Gentlemen:
It was a slow day at work today, so with time on my hands, I put together a spreadsheet to try to draw some conclusions as to the affects that velocity and rifling twist rate have on roundball revolutions per second (RPS).
The formula is RPS = Twist rate (in revolutions per foot) times velocity in feet per second. The feet cancel and you are left with revolutions/sec.
A given caliber is said to have an optimum rate of twist to achieve stabilization. We also know that larger balls do not need to be spun as fast as a smaller ball to achieve stability and accuracy. Most of our big bore (50 to 58 caliber) round ball rifles, with slow twist rifling, are loaded to achieve a hunting load muzzle velocity of 1500 to 1800 fps. You can see the range of RPS for such velocities ranges from about 250 to 350 or thereabouts. This velocity range also generally yields optimal accuracy. I therefore determined that 250 to 350 RPS is the optimal spin rate for a common 50 to 58 caliber ball. One can also see that, to achieve a similar RPS value with a 1/48 twist rifle of the same caliber, less muzzle velocity is required. This is also in keeping with the "conventional" wisdom that 1/48 twists can be very accurate, but generally require lighter loads. As an extreme example, you can see that a 1/20" barrel designed for shooting slugs will spin a roundball at a stable rate if the muzzle velocity is very low, something that we know from experience is generally true. At the other extreme, the "Forsythe" Style rifling, very slow 1 in 104" twist, is designed for stabilizing round balls with massive powder charges behind big roundballs for dangerous game.
1 in 20”
Velocity RPS
500 - 300
600 - 360
700 - 420
750 - 450
800 - 480
850 - 510
900 - 540
950 - 570
1000 - 600
1050 - 630
1100 - 660
1 in 48”
Velocity RPS
800 - 200
900 - 225
1000 - 250
1100 - 275
1200 - 300
1300 - 325
1400 - 350
1500 - 375
1600 - 400
1700 - 425
1800 - 450
1 in 66”
Velocity RPS
900 - 164
1000 - 182
1100 - 200
1200 - 218
1300 - 237
1400 - 255
1500 - 273
1600 - 291
1700 - 309
1800 - 328
1900 - 346
1 in 104”
Velocity RPS
1000 - 116
1100 - 128
1200 - 139
1300 - 151
1400 - 162
1500 - 174
1600 - 186
1700 - 197
1800 - 209
1900 - 220
1950 - 226
:grin:
It was a slow day at work today, so with time on my hands, I put together a spreadsheet to try to draw some conclusions as to the affects that velocity and rifling twist rate have on roundball revolutions per second (RPS).
The formula is RPS = Twist rate (in revolutions per foot) times velocity in feet per second. The feet cancel and you are left with revolutions/sec.
A given caliber is said to have an optimum rate of twist to achieve stabilization. We also know that larger balls do not need to be spun as fast as a smaller ball to achieve stability and accuracy. Most of our big bore (50 to 58 caliber) round ball rifles, with slow twist rifling, are loaded to achieve a hunting load muzzle velocity of 1500 to 1800 fps. You can see the range of RPS for such velocities ranges from about 250 to 350 or thereabouts. This velocity range also generally yields optimal accuracy. I therefore determined that 250 to 350 RPS is the optimal spin rate for a common 50 to 58 caliber ball. One can also see that, to achieve a similar RPS value with a 1/48 twist rifle of the same caliber, less muzzle velocity is required. This is also in keeping with the "conventional" wisdom that 1/48 twists can be very accurate, but generally require lighter loads. As an extreme example, you can see that a 1/20" barrel designed for shooting slugs will spin a roundball at a stable rate if the muzzle velocity is very low, something that we know from experience is generally true. At the other extreme, the "Forsythe" Style rifling, very slow 1 in 104" twist, is designed for stabilizing round balls with massive powder charges behind big roundballs for dangerous game.
1 in 20”
Velocity RPS
500 - 300
600 - 360
700 - 420
750 - 450
800 - 480
850 - 510
900 - 540
950 - 570
1000 - 600
1050 - 630
1100 - 660
1 in 48”
Velocity RPS
800 - 200
900 - 225
1000 - 250
1100 - 275
1200 - 300
1300 - 325
1400 - 350
1500 - 375
1600 - 400
1700 - 425
1800 - 450
1 in 66”
Velocity RPS
900 - 164
1000 - 182
1100 - 200
1200 - 218
1300 - 237
1400 - 255
1500 - 273
1600 - 291
1700 - 309
1800 - 328
1900 - 346
1 in 104”
Velocity RPS
1000 - 116
1100 - 128
1200 - 139
1300 - 151
1400 - 162
1500 - 174
1600 - 186
1700 - 197
1800 - 209
1900 - 220
1950 - 226
:grin:
