The 1/3 Second Rule for Zeroing a Rifle

Richard Mann, of the Empty-Cases blog, offers up an alternative to the standard 100 or 200 yard zeros for hunting rifles.

I discovered that regardless of the bullet, its velocity, or BC, with the proper zero it is possible to keep it within a six-inch impact zone out to the distance it can reach in one-third of a second. I also discovered that a full value wind of less than 10 mph will not push a bullet egregiously far outside of that six-inch circle if it only acts on the bullet for one-third of a second. And finally, I leaned that all big game bullets will still impact with enough velocity to induce necessary expansion all they way out to one-third of a second from the muzzle, regardless of caliber or construction. (This learning experience has occurred over an extended period and involved a lot of ordnance gelatin.) 

All you need to do to sight your rifle in with the One-Third Second Rule is determine how far the bullet will travel in a third of a second and sight your rifle so that the bullet will impact three inches below your point of aim at that distance. Your bullet will then cross your point of aim (POA) once, very near 25 yards, and again, at some distance short of the distance it will travel in one-third of a second. 

A ballistic program will give you all of this data but be mindful to input the exact distance that the center of your riflescope is mounted above the center of your bore. If you fail to do so, the ballistics program will give you bad info. (Barsness also explains this in his book in the chapter about the mythical 25 yard zero.) It will also give you bad info if you fail to input the exact muzzle velocity of your load. So, you will need a chronograph too.

Read the whole thing.