Left @ 300 yds - Right @ 1000 yds? WTH?

[Michael Eichele]

meichele,

Where have you read about spindrift limitations of 6" at 1000 yds? I'm curious since I've read over on the sniper's hide forum where 10" and even slightly more spindrift isn't uncommon with some bullets at 1000 yds. The thread on Sniper's Hide was posted somewhere earlier in this thread and it's a very long thread and read. In summary, one of the guys posting was somewhat of a military-experienced expert on the subject, and he's the source of my information on 1 moa (~10") or more spindrift possible at 1000 yds. 10" is 67% more than 6" spindrift, so I'm looking for closer agreement/consensus on potential magnitudes of spindrift at 1000 yds.

I did indeed have my scope mounted crooked on my 7mm Rem Mag and that's why I was initially seeing so much left to right drift. I'm confident I've got everything pretty well mounted straight with the world at this point. The excess left to right drift (above and beyond that attributed to spindrift) could still be due to slight misalignment of the scope on the receiver - so slight that I can't eyeball it. I'm not going to mess with my setups anymore at this point. Now that I've confirmed and quantified my left-to-right drift, I can make provision for it. If I was shooting at prairie dogs, I'd probably refine my setups further, but hits on big-game sized animals shouldn't be compromised due to my bullet drift now - out to at least 1100 yds.

My tentative plan is to leave the rifle hitting on poa at 1000 yds and allow the leftward hits at the closer, mid-ranges. I will shoot both rifles at the 600 - 700 yd range to confirm my POI's and then see if I have more or less leftward POI than I'm currently getting at 300 yds. I can live with 4 inches left at the intermediate ranges. I'd rather be 4 inches left at midrange than 10" or more right at 1100 yds. My rifles both peter out velocity and energy-wise by ~1100 yds.

What you have read is correct. I think you missunderstand my statement. Your particular loads in your particular rifles shouldnt be much more than about 6".

There are ALOT of variables that go into figuring spin drift. Velocity, air density, BC, twist of barrel, ect....AND their relationship to eachother.

Please understand I was not saying that all small arms ballistics yield 6" rather it was generated by a ballistics program based on what info you did provide. I obviously dont know your twists in your barrels so it was a guess. They could be more. But not much more. Still your real world spin drift is alot less than where your bullets impacting at the time of your first post.

I have seen 10-12" of spin drift in my old test 308 at 1K depending on conditions AND the load used. This was concistent with ballistic software and other calculations. I have also seen much smaller results from other of my rifles and loads. Again concistent with calulators and calculations.

For me to say that bullets are limited to 6" @ 1K would be erroneous at best. The fact is that it will be different from load to load and barrel to barrel. Now similar barrels with similar loads will be VERY close but drastically different rifles and loads will be much different from eachother.

I hope that clears up my original comments!

PS:

One more thought.

The point of my first post wasnt to tell you exactly how much SD you are getting rather it was an estimate to illustrate that SD was not the one and only reason you were seeing such a dramatic right impact down range. If I missled you in any way I humbly apologize.

 

[Michael Eichele]

Deteted

(copied and pasted to the post above.)

 

[Pdvdh]

Thanks for the follow-up meichele. I appreciate your comments as it helps gel my knowledge regarding relative magnitudes of spindrift. I'm led to believe it's just about impossible to predict spindrift precisely, however from my perspective, it's worth testing long range rifle set-ups in the effort to quantify how much is present.

This last time I shot at 990 yds was in a light snowfall and the snow was falling straight down. No wind at all that I could detect. So I'm pretty confident about the amount of bullet drift I'm getting with these two rifles. My drift distances are now small enough that I don't know how, with any certainty, I could dertermine how much is spindrift versus a left to right drift caused by either the rifle set-up, or the pilot (me).

 

[royinidaho]

meichele,

You got me hooked here. However, I am pretty darn tired of getting 'ready'. I think I need another pill.

I'm well on the way to wearing a very good barrel out getting ready. I've had bullet availability problems, plus a little tinkeritus..... Now I have spin drift to worry about.

Within the next week I'll have it determined as the weather is becoming more agreeable.

So.......how does one ever so slightly turn the scope ever so slightly in the proper direction and get it right with out burning up a couple of days traveling that 2000 yd trek and burning up way more barrel.

This process seems the most reasonable to me as I very strongly desire to minimize the amount of "stuff" I have to think of immediately prior to the shot. For many of my situations there just isn't enough time to mess with all the charts and stuff. Its range, click for elevation using BDC turret markings, hold for wind and shoot. It works well. But now I'm just getting into the beyond 800 yard yote shooting distances.

 

[Michael Eichele]

So.......how does one ever so slightly turn the scope ever so slightly in the proper direction and get it right with out burning up a couple of days traveling that 2000 yd trek and burning up way more barrel.

I do it by loosening one side of my rings and tightening the other. It takes time and experimentation as to how much and how each set of rings works. Some rings are more cooperative than others. I had a rifle/scope that I scrwewd with over and over and couldnt get her right. She ended up level and I left it alone, learned how much drift was present and used the approriate addition or subtraction to any wind compensation. Sometimes it is just easier to use some kentucky windage. To simplify things, you can just learn how much you have and with that knowledge you can just compensate a bit.

Another time I got lucky and got the offset on the first try!

No need to overcomplicate things or burn out a barrel. Learn it, dope it.

 

[royinidaho]

No need to overcomplicate things or burn out a barrel. Learn it, dope it.

That's what I figured. I usually end up doing that any way and it has served me well.

Thanks..

 

[Pdvdh]

Re: You're not gonna believe this

I was more or less dumbfounded last night. I recently purchased Patagonia Ballistic's Loadbase 2.0 ballistic program and software. You end up receiving both their Desktop PC program and their PDA or PPC program. One is for use at the house and one for use on the handheld PPC out in the field. I purchased a Dell Axim X51V PPC off of fleaBay and soon had both the Desktop PC and PPC versions of Loadbase 2.0 up and running.

So last night I get to an option on my PPC version that allows for the inclusion of Coriolis effect on my POIs. You have to enter the latitude of your shooting location, which for me is 60.4 Degrees North, in order for the program to calculate the correct amount of Coriolis drift. Turns out that the farther you're located from the Earth's equator, the greater the Coriolis affect and drift on your bullets. Well I'm up here in Alaska so I'm a fair ways from the equator.

I enter the 60.4 Degrees North latitude and then activate the Coriolis function of the software, and I'll be danged if it didn't contribute 4.5 inches of rightward drift to my bullet's POI at 990 yds, which is the distance I shot both my 7mm Rem Mag and 300 WM in order to quantify my left to right drift.

If you recall from my earlier post in this thread, I reported that with both rifles sighted to hit dead on left-right wise at 988 yds, I found that my 7mm Rem Mag was then hitting 4" left at 300 yds , and my 300 Win Mag was hitting 3 1/2" left at 300 yds.

Well spindrift is supposed to cause roughly 0.9 moa rightward drift at 990 yds. 0.9 moa spindrift equates to ~ 9 1/4 inches of rightward drift @ 990 yds. And in order to compensate for spindrift alone at 300 yds, I would have to adjust my POI ~ 0.9 moa left, causing my bullets to strike about 2 3/4" left at 300 yds. If you'll remember, in my prior post on this thread, I concluded that spindrift (2 3/4" left POI @ 300yds) appeared to have accounted for the majority of my rightward drift at 990 yds, but that I didn't have an explanation for the additional left adjustment required at 300 yds for both my 7mm Rem Mag (4" left @ 300 yds) and my 300 Win Mag (3.5" left @ 300 yds).

NOW --- let's calculate how much additional leftward compensation is required at 300 yds in order to allow for the 4 1/2" of additional Coriolis drift @ 990 yds. 4 1/2" right drift at 990 yds would require an additional 1 3/8" leftward shift of POI at 300 yds.

So here's the math for my 7mm Rem Mag: The theoretical leftward POI required @ 300 yds to be dead on at 1000 yds = 1 3/8" (Coriolis drift) + 2 3/4" (spindrift) = 4 1/8" left POI @ 300 yds. ****! My 7mm Rem Mag actual POI was ~ 4" left at 300 yds after having been sighted in to strike dead on @ 990 yds. The agreement is within 1/8".

And here's the math for my 300 Win Mag: The theoretical leftward POI required @ 300 yds to be dead on at 1000 yds = 1 3/8" (Coriolis drift) + 2 3/4" (spindrift) = 4 1/8" left POI @ 300 yds. Great Horny Toads! My 300 Win Mag actual POI was ~ 3 1/2" left at 300 yds after having been sighted in to strike dead on @ 990 yds. The theoretical to actual measured difference is 5/8".

Take it or leave it. As for me, I'm a believer! I went to great lengths to ensure my scopes were mounted plumb with my barreled receiver and properly aligned with gravity (not canted) at the moment I squeezed the trigger. I picked a dead still day (light snow falling straight down from my shooting position all the way out to 990 yds). I was at a bit of a loss for the additional ~ 3/4" - 1 1/4" of leftward adjustment required at 300 yds in order to hit dead on at 990 yds. But I dropped the subject because that wasn't substantial enough for me to spend anymore time worrying about it. Now I blunder into a scientific explanation based on known and proven Coriolis drift! I was blown away to realize how close the theoretical drift matched my measured drift. And understand I measured my drifts before I calculated them, so there was no hanky panky going on here.

North of the equator the Coriolis drift is rightward. South of the equator the Coriolis drift is leftward. The farther you're located from the equator, the greater the Coriolis drift. Up here in Alaska I now know we can experience 4 1/2" Coriolis drift at 990 yards. If you're shooting at the equator, the Coriolis drift is essentially zero.

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.

 

[davewilson]

are any of you ready to position the crosshairs a bit to the left yet? i set mine so the bullets hit about 1/2" to the left at 100 yards,of a verticle line at 25moa up.

 

[Michael Eichele]

Re: You're not gonna believe this

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.

 

[royinidaho]

Deleted a bunch of stuff.......

phorwath,

Could you please give me some numbers so's I can twist my scope?

How much cori adj: I'm between 42* and 46* for most shooting activities.

I've been bustin'' rocks, etc, out to 1200 but never on the first shot.

Usually after two windage adjustments I'm spot on. Elevation is spot on. Don't seem to have much of a problem under 800 yds. Two warm up shots ain't gonna cut it.

I'm always too far right and attributed it to insufficient windage adjustment. Wind is always from my left.

Thanks

 

[Pdvdh]

Roy,

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.

 

[jmason]

phorwath,

Thank you very much for this post and following through to a solid conclusion. I'm saving this post so I can refer to it later!!!!

 

[royinidaho]

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.

Typo????

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.

lightbulb 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?

 

[Pdvdh]

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.