Page 1 of 1


Posted: Sun Nov 26, 2017 8:12 pm
Let me pass on some shooting trivia.....I realize that there are a bunch out there that are familiar with this stuff and are more familiar with it than I, but some may not be, okay. And that is ‘Drift’ as in bullet drift. I have seen in the past where some will write and say that bullet drift is caused by the spin of the bullet from the effects of the bore riflings. Okay let me qualify everything before I continue - this is just the way that I understand things and ’I don’t know anything for sure’ as no one else does either. This is just a philosophical point.
But based on my Inertial Guidance and navigational equipment experience of aircraft and missiles and physics. I will just delineate some things.
In the past when shooting writers would talk about ‘Bullet drift’, they would quite often say that bullet drift was caused by the spin of the bullet. There is some, but very little caused by the spin of the bullet, because the drift caused by bullet spin is only in effect while the bullet is falling a small distance caused by gravity in its travel. It is exactly like a curve ball in base ball but not as apparent. Most of the drift is caused by the Coriolis Force and normally and properly, I think, referred to as the ‘Coriolis Effect’, because there is no real force causing any drift at all. It is an illusion making one see an apparent drift where actually it is the surface of the earth that is drifting as to the right in the northern hemisphere and to the left in the southern hemisphere and no drift if shooting east or west at the equator. Now if one shoots north or south from the equator there will be a slight and I mean slight apparent drift because the bullet will be entering either the northern or southern hemisphere. And then...and then, there will be such a slight difference in this amount of drift, I doubt if any common computer will even have an entry box for this very slight difference input. I have been out of this business since I retired in 94. As far as I have seen the common military shooting hand held computers used today are only concerned with the latitude, north or south as an input not the compass direction of the shooting. Maybe some military types here can input on this. At our normal latitudes here in the state one can actually see Coriolis bullet drift with a high powered scope. I have the formulae around here some place and calculated the Coriolis drift for here at my latitude. It was about 1.24 feet for a two mile travel as I remember. The average speed of the projectile during its travel and the latitude are usually the only inputs needed....the larger the latitude the greater the Coriolis drift. It doesn’t make any difference whether it’s a 50 caliber bullet or an F-104 it will be the same. But with aircraft the speed, the latitude, the altitudes and attitudes are usually constantly changing so the systems computers have to be constantly working to tell the pilots where they are at. The one in the F-104 could keep track of things as a pilot would go over one of the earth’s poles...because it was supposed to be able to zip over the north pole and drop an atomic bomb on Moscow. All of the guidance systems at that time couldn’t do this. I assume that most today can.
It will be different at what latitude one is shooting at but roughly if one is at a latitude similar to that of southern Colorado the Coriolis drift would be about the distance that you are shooting divided by two miles times 1.24 feet, and if the average traveling velocity is similar. Think of it as zero at the equator and maximum at the poles. And the diameter of the earth is roughly 7 miles less at the poles than at the equator. And the earth is pear shaped....this is what I have been told, I have never measured it. :) But one can actually see this with a high powered scope and on a windless day . I’ll grab a calculator and see what I may get for a hundred yards. I get 0.035 feet of Coriolis drift . Now you realize that if you have your rifle sighted in for a hundred yards, you should hit as sighted in and see no drift evidence on the target, but just as you see the bullet traveling, and if everything is set up right on your rifle and you are holding it level, if you shoot again at 200 yards, you should see that your bullet should strike roughly 0.035 feet to the right on the target. As I remember that figure was at two miles and not one, I wouldn’t bet my life on it, okay. It would be nice if there are some others that maybe more familiar with this and put in their input.