Bullet spinning energy - is it significant?
I said earlier
I assert that any amount of spin, excessive or not, detracts from muzzle velocity. Of course, some spin is necessary, as we've been discussing. I will calculate the bullet energy consumed by spin and post it later...
So, here it is. The needed formulas tying the spin energy to bullet mass, diameter and spin speed rely on the
moment of inertia of the bullet. The moment of inertia about the spin axis is a property of the shape of the bullet and the density of the material. I approximated the shape as a cylinder and took the material to be solid lead (density is 11340 kg/m^3 = 11.34 g/cc = 185.83 g/cu.in. = 2867.7 gr/cu. in.). Here are the formulas, in which "W" is the energy or work just to spin up the bullet:
In the formulas, "M" is the mass of the bullet, "r" is the radius of the bullet (half the caliber diameter) and omega is the spin speed, as explained. This all has to be worked out in consistent units. I have a spreadsheet for that, if anybody wants me to upload it.
It turns out that the spin energy carried by the bullet to the target is small, less than 1 %, compared to the linear speed energy, which is technically called the translational energy. Here are three examples sort of covering some extremes.
A 180 grain .308 inch diameter lead bullet travelling at 2620 fps out of a barrel with a twist of 12 inches/turn has:
Bullet translational energy of 3719 Joules = 2743 ft*lbf
Bullet rotational energy of 12.1 Joules = 8.92 ft*lbf.
So the ratio of rotational to translational energy is 0.325 %.
Bullet length, effective (cylinder approximation): 0.842 inches
A 300 grain .452 inch diameter lead bullet travelling at 1500 fps out of a barrel with a twist of 30 inches/turn has:
Bullet translational energy of 2032 Joules = 1499 ft*lbf
Bullet rotational energy of 2.28 Joules = 1.68 ft*lbf.
So the ratio of rotational to translational energy is 0.112 %.
Bullet length, effective (cylinder approximation): 0.652 inches
A 62 grain .223 inch (5.56 mm) diameter lead bullet travelling at 3002 fps out of a barrel with a twist of 7 inches/turn has:
Bullet translational energy of 1682 Joules = 1240 ft*lbf
Bullet rotational energy of 8.42 Joules = 6.21 ft*lbf.
So the ratio of rotational to translational energy is 0.501 %.
Bullet length, effective (cylinder approximation): 0.554 inches
These energies carried by the spin of a bullet do not include the extra friction of the faster spinning bullet lost in the barrel or to the air.
Zoogster pointed out these effects earlier. This friction energy is lost as heat, not delivered to the target.