bullet drop and scope leveling

[Bart B]

I wasn't using a scope when I did my drift test shooting at 100 and 1000 yards. I used aperture sights with a spirit level on the front one. When installing the sights, a level was placed on the rear sight's elevation slide then the front sight (mounted on a barrel band) was rotated until it's level matched the one on the rear sight. The error between the rear sight being level such that it elevated in the true vertical was no more than 5 MOA.

The spirit level indicated a 20 MOA cant when its bubble was about 1/8th inch off center. All shots were fired with the bubble no more than 1/32 inch off center (about 5 MOA cant). The biggest cant error between the vertical sight axis and the true vertical was therefore no more than about 10 MOA. The horizontal deflection caused by a 12 MOA (2/10ths degree) cant for a 155-gr. bullet's 2 inch drop at 100 yards from a .308 Win. is about .007 inch; at 1000 yards it's about 1.1 inch for its 325-inch drop; elevation was about 5000 feet and muzzle velocity about 3025 fps.

At 100 yards, the 15-shot group was about 3/8ths inch and about 9 inches at 1000 yards. Both were centered horizontally. This is why I think whatever drift from precession there was (is?) didn't make itself visible because it was negligible.

 

[Michael Eichele]

Bart, I believe you. I also believe either some minor wind or a SLIGHTLY canted sight or both cancelled out need for correction for this drift. I have some rifle/load combos that drift about 3" at 1K and others that drift over 10" at sea level in sub zero temps. At 6000' in 60 degree air when I am sheep hunting this drift is reduced to some degree but for my rigs and loads I still correct for it past 600 yards.

If you have a proper stability factor and shorter time of flight such as will be seen at 5000' and with an over 3000 FPS MV this drift will be cut down quite a bit. That still doesnt negate the fact that <font color="red"> others </font> wont see it. It is going to depend on how the equipment is set up, stability factor based on twist, velocity bullet dimensions and air density. Also air density's effect on the overall flight and its effect on flight time. If you dont see it then there is obviously no need to correct for it. Some will some wont. If it happens the only 2 thing you can do is add or subtract its value from the wind at hand or cant the scope to the left a fraction of a degree for a right handed twist barrel and correct for raw windage.

Please understand I am not arguing with you. I know that spin drift doesnt always show up for 1 reason or another as it didnt in your case. The point I am trying to make is that whether or not a shooter "sees" it or not for various reasons that cancel the need for correction, it still happens and in other cases and circumstances other shooters will see it show up and have to correct for it.