RPM of a speeding bullet
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lysanderxiii
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This is known as tractability. If you look at the equation for tractability, the cosine of the angle of the trajectory (theta) is in the bottom. This means as the angle of the trajectory decreases the tractability factor (f) increases. The tractability factor needs to be above 5.7 in order for the nose of the projectile to follow the trajectory curve.
The angle of departure for .30-06 and 7.62 NATO is in the range of 45 to 50 mils for 1000 yards, about 8 to 10 degrees. Tractability isn't a problem for such low angles. When it is said that tractability is a problem at long ranges, they mean angles of departure in the 20 degrees or greater range. 1000 yards is relatively short range it's only 1/10 the maximum achievable range of these bullets.
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8 to 10 degrees (480 to 600 MOA) line of fire above a horizontal line of sight to zero for a 308 at a thousand yards?
Did you mean 45 to 50 MOA instead of mils?
Sierra's software calculates a .30-06 shooting their 180-gr. SBT out at 2700 fps at about 36 degrees angle above a horizontal line of sight has a maximum range of about 6000 yards.
Did you mean 45 to 50 MOA instead of mils?
Sierra's software calculates a .30-06 shooting their 180-gr. SBT out at 2700 fps at about 36 degrees angle above a horizontal line of sight has a maximum range of about 6000 yards.
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lysanderxiii
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That number comes from Hatcher's Notebook, page 401 & 402
The angle of departure for M1906 Ball is 49 mils, for M1 Ball, 41 mils
I don't know what I did to get 10 degrees, but that is wrong....
In any case, even when shooting out to 1000 yards we are talking very low angles of departure.
The angle of departure for M1906 Ball is 49 mils, for M1 Ball, 41 mils
I don't know what I did to get 10 degrees, but that is wrong....
In any case, even when shooting out to 1000 yards we are talking very low angles of departure.
In my copy of Hatcher's Notebook, page 402 lists the 30-06 M1 bullet leaving 2600 fps requires a departure angle of 41.2 minutes of angle to zero at 1000 yards.
That parallels well with my experience shooting Garands at 1000 yards needing 42 to 43 clicks up from boresight to zero M72 match ammo in .30-06 versions and M118 match ammo in 7.62 NATO versions.
That parallels well with my experience shooting Garands at 1000 yards needing 42 to 43 clicks up from boresight to zero M72 match ammo in .30-06 versions and M118 match ammo in 7.62 NATO versions.
I've run those number on various firearms. The numbers at ridiculous. For instance a 1;7 twist ar with a muzzle velocity of 3000 fps comes out to about 308,571 rpm. I don't remember exactly. It's some where in that area.
NHRA Pro Stock engine about 10,000 rpm
Modern jap motorcycle engine about 14,000 rpm
Some electric motors 20,000 rpm
Jet engine turbine about 40,000 rpm.
I can't think of anything else man made or in nature that spins even remotely close to as fast.
NHRA Pro Stock engine about 10,000 rpm
Modern jap motorcycle engine about 14,000 rpm
Some electric motors 20,000 rpm
Jet engine turbine about 40,000 rpm.
I can't think of anything else man made or in nature that spins even remotely close to as fast.
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Vern Humphrey
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Just a quick calculation tells you that Hatcher used MOA, not MIL.
41 MILs X 36 inches = 1476 inches of drop at 1000 yards
41 MOA X 10 inches = 410 inches of drop at 1000 yards
A .308 caliber bullet will drop somewhere around 400 to 500 inches at 1000 yards.
41 MILs X 36 inches = 1476 inches of drop at 1000 yards
41 MOA X 10 inches = 410 inches of drop at 1000 yards
A .308 caliber bullet will drop somewhere around 400 to 500 inches at 1000 yards.
BobWright
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I once calculate the RPM of various bullets, based on muzzle velocity and rifling pitch. Most rifle bullets exceeded 100,000 RPM, while most handgun bullets were in the 35,000 RPM. Thus, the shorter the bullet for caliber, the slower the spin, while longer bullets require faster spin rates.
The terms "understabilization" and "overstablization" while apparently non-existant, are actual terms used in ballistics. If the spin is too slow, a bullet will tumble in flight, hence lose any aspect of accuracy. A bullet that is "overstablized" or spun too fast, will not follow the trajectory's arc, but will tend to remain constant with the angle of departure. Such overstalization results in loss of downrange performance as well as a loss of accuracy.
An overstabilized bullet will display key holing downrange on a paper target.
Granted, these are very long range studies conducted for overhead machine gun fire and heavy artillery, but the application is the same. Regardless of the caliber, .30" or 3", projectiles having the ballistic coefficient of 1.0 behave in the same way.
Bob Wright
The terms "understabilization" and "overstablization" while apparently non-existant, are actual terms used in ballistics. If the spin is too slow, a bullet will tumble in flight, hence lose any aspect of accuracy. A bullet that is "overstablized" or spun too fast, will not follow the trajectory's arc, but will tend to remain constant with the angle of departure. Such overstalization results in loss of downrange performance as well as a loss of accuracy.
An overstabilized bullet will display key holing downrange on a paper target.
Granted, these are very long range studies conducted for overhead machine gun fire and heavy artillery, but the application is the same. Regardless of the caliber, .30" or 3", projectiles having the ballistic coefficient of 1.0 behave in the same way.
Bob Wright
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