Location: The rifle range, or archery range or behind the computer in Alaska
Re: You're not gonna believe this
Originally Posted by phorwath
I've read posts from others that claim spindrift and Coriolis drift are of no use and have no application for the long range hunter. They'd have their work cut out convincing me. Like I said... consider me a believer.
Amen to that.
Those that never experianced either havnt shot far enough or their scopes are accidentally offset to their benefit.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
At Latitude 44 Degrees North, MV of 3056 fps, BC of 0.667, Altitude of 290 feet, I get 2.3" Coriolis drift @ 800yds, 3.6" Coriolis drift @ 1000yds, and 5.2" Coriolis drift @ 1200yds. The Coriolis drift will be to the right. You have less drift down there in the real world because my Latitude up here is 61 Degrees.
You could expect about 8-9 inches of left to right spindrift on average @ 1000 yds. Spindrift increases at an increasing rate at greater distances. The only way I know to quantify your spindrift is to make sure your equipment is all plumb with the world and then shoot like I did on a windless day at extended ranges. If you shoot a reasonably tight group you'll be able to ID and measure your left to right drift. By subtracting Coriolis drift from your total measured drift, you'd be ID'ing the spindrift from your specific rifle, load, and barrel.
Make sense? Nothing's free. I spent a buttload of time to come to my conclusions, so expect to put in some time if you want to quantitate your spindrift. If you don't want to go throught the effort, then count on 8-9 inches spindrift at 1000 yds and 3.6" Coriolis drift. Spindrift will always be right-ward drift with a right twist barrel, and Coriolis drift will always be right-ward drift on our side of the equator.
Let'me know if you've got any further questions. I'm not claiming to be an expert at this but I'll only offer information after I'm pretty confident I've got it properly figured out myself.
Last edited by phorwath; 12-19-2008 at 09:22 PM.
Reason: Correct direction of Coriolis drift - to the Right
Make sense? Nothing's free. I spent a buttload of time to come to my conclusions, so expect to put in some time if you want to quantitate your spindrift. If you don't want to go throught the effort, then count on 8-9 inches spindrift at 1000 yds and 3.6" Coriolis drift. Spindrift will always be left to right with a right twist barrel, and Coriolis drift will always be left on our side of the equator.
Thanks for your diligence. Yup, there's no free lunch, but its always nice to have an idea of what the heck you're chasing!
@ 1100 yds + I notice that I have yet to guess the wind correctly, even when its very low. I'm usually just a bit to the right.
A nerdy question: does Coriolis exhibit itself when shooting east and west (90 and 270 degrees)?
Your next assignment will be to trek to the true north pole do the same test always shooting south.
OK, here's what I'm gonna do.
I'm gonna tilt the scope per DaveWilsons recommendation. Then for cori tweaking I'll ignore it out to 600 yds.
From 600 to 900 I'll give it one click left
Beyond 900 I'll give it 2 clicks left.
Then make the windage adjustment from there. Sound reasonable?
Last edited by royinidaho; 12-19-2008 at 11:01 AM.
Reason: Gettin more goof ball ideas......
Thanks for catching my error and yes that was a typo. Coriolis drift north of the equator is always to the right, no matter the direction of fire (north, south, east, or west) according to the output I'm receiving from Patagonia Ballistics' LoadBase 2.0 software. I don't have the scientific explanation for this, but it's always the same amount of right-ward drift for the Latitude location and the same rifle & load combination.
Coriolis drift also causes vertical drift if you're shooting east (90) or west (270). No vertical Coriolis drift is caused if you're firing directly north or south. However this vertical Coriolis drift is only about 1/2 the amount of the horizontal Coriolis drift. For example, if your shooting direction is due east (90 degree azimuth) at 44 degree Latitude, the bullet POI is calculated to be 1.79" higher @1000yds due to Coriolis effect. On the contrary, if you're shooting due west (270 degree azimuth) the bullet POI is calculated to be 1.79" lower @1000yds due to Coriolis effect.
I've read of people canting their rifles slightly to counter the effects of spindrift and Coriolis drift, which is basically what Dave is doing by tweaking the scope slightly clockwise in the rings. Other's add from 1/2 to 1 moa right-ward dope when shooting at 1000 yds. Rotating the scope slightly in the scope rings can be done but it's a bit of a hit or miss proposition, meaning you'll have to fire and test after your scope adjustment to see if you tweaked it too little, just right, or too much. I'm considering just leaving my two outfits hitting 3.5 to 4" left at 300 yds, meaning they'll be dead on at 1000 yds in windless conditions, for purposes of harvesting big game animals. From 250 to 700 yds I'll be hitting slightly left, but not enough so to cause a miss at those distances unless I'm taking head shots, or shooting at mice.
Let us know what you decide and how it works for you. Now that I understand these two sources of drift better, I too am in the process of deciding how I want to compensate for them.