extreme spin drift

[sport20]

hi
I'm just wondering why after I shoot pass 300 yrds everything hits more and more to the right. at 920 yrds I'm hitting about 18"s to the right. is it possible for that much spin drift.any help would be appreciated
thanks
load data
7mm rem mag. sender
rem cases federal match mag primers
70 gr H1000
3.437" boal
168 gr. bergers

 

[Scottytuned]

My first thought wouldn't have been spin drift. I guess knowing a little bit more could help.

Is this a one time event where it could have been wind patterns or does this occur frequently?

Is it directly right or is there a little vertical movement as well? I'm thinking it may be something of the shooter (no offense) such as canting the gun/scope. Is it possible the scope isn't perfectly vertical? I know when I run into things like this the first thing I do is hand the rifle to someone else to try to rule out my error. Could be a lot of different things so hard to rule for sure.

 

[el matador]

Things that make your bullet go to the right:

Coriolis
Spin drift
Wind
Scope reticle not level
Scope not tracking parallel with reticle

Coriolis might add a few inches to your error, depending on which direction you're shooting. Spin drift is around 6" at that range. If you're not using a ballistic calc that compensates for spin drift you could sight in about 1/2 moa to the left. This will offset most of your spin drift out to about 1000 yards. A 5 mph wind will make your bullet drift off course about 18" at that range. Having your scope canted 5 degrees will also give you about 18" of error. My guess is that its a combination of these things. You can check for scope error by doing the "tall target test": http://www.longrangehunting.com/forums/f17/tall-target-test-video-comments-155589/

 

[sport20]

if I dial 2moa left at 920 yards im right on target. within 6 "s. so I come back and shoot at 300 yrds and I'm 6" to left. I'm confused because I took a lot of time setting up scope using the wheeler kit and levels. no wind while testing

 

[bigngreen]

You need to do a tall target test to level your turret movement, the levels just set up the od of the scope to the action. You also need to shoot with a level on the gun, it does not take much cant to throw you out a lot!

 

[Pdvdh]

Re: Excessive Rightward Drift

I once experienced excessive rightward drift with one rifle, compared to a similar rifle in side by side tests shooting at 1000yds.

After examining everything closely, I saw that the scope rail alignment on one of the rifles was left or right of the barrel alignment (can't recall which way now). I corrected the crooked scope rail and got it running parallel to the barrel by running a straight edge along the side of the rail and checking the alignment at the muzzle off of both sides of the one-piece scope rail.

That corrected my problem. I never understood the physics involved in creating the excessive left to right drift. But after I mounted the scope rail parallel to the barrel, the rightward drift downrange was the same as my other rifle, and matched the predicted rightward drift, accounting for both spin drift and coriolis drift.

Now when I mount a scope rail, I always check it with a straight edge which extends all the way down to the muzzle. My scope rail was off far enough that I initially identified it visually, just looking down on the scope from the top. So it was off quite a bit.

But there are a number of additional sources that can cause this, which others have identified.

 

[BergerBoy]

hi
I'm just wondering why after I shoot pass 300 yrds everything hits more and more to the right. at 920 yrds I'm hitting about 18"s to the right. is it possible for that much spin drift.any help would be appreciated
thanks
load data
7mm rem mag. sender
rem cases federal match mag primers
70 gr H1000
3.437" boal
168 gr. bergers

I have a 1-7 twist in my 308 and I have closer to 15 in. at 1000 yds.

 

[BergerBoy]

You need to do a tall target test to level your turret movement, the levels just set up the od of the scope to the action. You also need to shoot with a level on the gun, it does not take much cant to throw you out a lot!

Bigngreen has a great idea..... I use a level on my rifle then lock it in and then use a plumb-bob line to line up my vertical reticle to help eliminate cant when I mount my scopes. It can be a pain in the ***.

good luck

 

[sport20]

hi guys
thanks for all your input. I rigged up a level on scope when I was shooting at 100 yrds and I was canting the gun to the right. I never had a chance to try long range but I'm sure this was the problem

 

[Pdvdh]

Re: excessive horizontal drift

That would be about the simplest, and perhaps most common, cause for excessive horizontal drift.

 

[spurlockda]

It's really beneficial to do as someone mentioned earlier and get some scope levels that you attach to your scope mount rings. You would be surprised how many folks cant their rifles. Once you get the levels and add that one more layer of consistency to your efforts, you will see advances in your skillset with marked increases in your own accuracy. "Consistency is accuracy!"

 

[WildRose]

hi
I'm just wondering why after I shoot pass 300 yrds everything hits more and more to the right. at 920 yrds I'm hitting about 18"s to the right. is it possible for that much spin drift.any help would be appreciated
thanks
load data
7mm rem mag. sender
rem cases federal match mag primers
70 gr H1000
3.437" boal
168 gr. bergers

Run your numbers for 920yds through a good ballistic program including your twist rate and tell us what it comes out to.

When you know what is predicted then add/subtract you can start diagnosing the problem.

What scope are you using and is it parallax adjustable? Parallax can certainly add noticibly to windage errors beyond 600yds.

 

[RMulhern]

It's impossible to ignore the Laws of Physics!

Even if you are not possibly canting the rifle and if you were say zeroed dead on at 200 yards with elevation and windage....when you move back to 1000 yards without taking any left windage (assuming a right hand twist) your POI in a no wind condition will be to the right of your intended POI!

Spin drift is the culprit! Runs about 14" with the .308 Winchester shooting military match ammo.

 

[John McNab]

Have had the identical problem with both old target rifles converted to scope use (home gunsmiths working on Omark 44's) and Remington hunting rifles. Problem was incorrect alignment of scope bases or rails and/or mounts and barrell as per Porwath above. Get a 1" diameter length of dead straight aluminium tube (extendable sign poles are made from good quality tube) cut it the length of you barrell + a foot or so. Get a good set of 1" mounts and a good base you know are true, and mount the the whole lot on the rifle with the tube end just overhanging the muzzle. Use your Wheeler to set the rifle up dead straight up and down. Observe from both muzzle and butt end looking at the alignment of the tube to the barrell, even a tiny side to side miss alignment will be obvious, and the cause of the problem. Leave the rifle set up like that and take to your gunsmith so he can see the problem, he'll sort it by over drilling the mount screws in the right alignment and threading them for the next size screws. Have once seen it caused by the barrell thread cut not square to bore, but thats really rare.