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

[4xforfun]

This may have been covered....if I missed it, I am sorry.


Now, you say that your scope is mounted correctly as far as scope cant and alignment.


Now, with your gun FLAT on the rest and with a plumbed line on a target at 100 yards, shoot a group......dial up 25 moa, shoot another group......does you scope go strait up, putting the group ON THE LINE.

In other words, is the INTERNAL adjustments you are making with your scope equal EXACTLY what you are looking for on target. Is the dialed up group off to one side of the line......1 inch @ 100 = 10 inches at 1000. I have seen a scope that, when dialed up, went to the left. Someone, somewhere, told me that this was not possable. IT WAS!! And that scope was on strait and mounted correctly. But, it was a "bargan" brand scope!!


Just thinking more about this....you shouldn't even need to fire any groups. Just make sure the gun DOES NOT MOVE when you are making the adjustments. You should be able to watch the scope track up the line as you dial up. I know that it is easier with a flat bottom BR stock than a hunting stock.

I guess to be sure I would shoot actual groups. That would be my BR tendancies rearing their ugly head again.

 

[davewilson]

great post. every scope has the potential to not track perfectly verticle. maybe not the Sightron, but almost any other brand can. the erector tube that the turrets are pushing against are round. the farther you get away from the center of the erector tube, the more you move sideways because of the curved surface. this will probably only show up towards the end of the verticle adjustments. this can also happen when in the outer limits of horizontal adjustment, you would start changing the verticle position.

 

[Jon A]

Yes, a scope should be checked for a canted reticle before a shot is ever fired (especially if it's a Leupold), long before one worries about things like spin drift. 5 minutes with a $50 collimator can save hundreds of dollars worth of time and ammo chasing down problems that could have been diagnosed in one's living room.

 

[jmden]

"Just thinking more about this....you shouldn't even need to fire any groups. Just make sure the gun DOES NOT MOVE when you are making the adjustments. You should be able to watch the scope track up the line as you dial up. I know that it is easier with a flat bottom BR stock than a hunting stock."

If you use a bipod for the front rest shooting off a bench, you can level the rifle using whatever device you have and then check the tracking. The bipod will hold the hunting rifle more steady (in one place without shifting) for this process than front bags/rest will.

 

[Pdvdh]

I don't twist turrets for elevation or windage, so all I'm concerned about is getting my scope reticle mounted perpendicular to the receiver, and the scope tube mounted parallel to the barreled receiver. Once I've accomplished that, then as long as I don't cant the rifle clockwise or counterclockwise while shooting, I should be good.

But I do understand the recommendations to ensure vertical tracking for those that twist turrets.

 

[4xforfun]

"Just thinking more about this....you shouldn't even need to fire any groups. Just make sure the gun DOES NOT MOVE when you are making the adjustments. You should be able to watch the scope track up the line as you dial up. I know that it is easier with a flat bottom BR stock than a hunting stock."

If you use a bipod for the front rest shooting off a bench, you can level the rifle using whatever device you have and then check the tracking. The bipod will hold the hunting rifle more steady (in one place without shifting) for this process than front bags/rest will.

I guess I forgot about the Harris Bi pods....they will keep you level and solid on the bench. I have been using the B-Square "Roto-Tilt" for hunting. It isn't very good for pure bench shooting, but is fantastic on unlevel ground and for "tracking" moving animals.

As far as my earlier post...the scope was level, and the rifle was not canted....the scope tracked up and to the side.

 

[davewilson]

4xforfun;...the scope was level, and the rifle was not canted....the scope tracked up and to the side.

this is why if you use the turrets, you have to adjust the scope so it tracks vertically. the direction the turrets track and the crosshairs are not always on the same plane. just another reason i'm gonna have multiple dots on the crosshair on my next setup.

 

[jwp475]

Dave, what scope did you have that tracked in a canted plane. Just curious

 

[4xforfun]

4xforfun;...the scope was level, and the rifle was not canted....the scope tracked up and to the side.

this is why if you use the turrets, you have to adjust the scope so it tracks vertically. the direction the turrets track and the crosshairs are not always on the same plane. just another reason i'm gonna have multiple dots on the crosshair on my next setup.

I wouldn't stress about bad a tracking scope. I have seen this only once...on an el-cheapo. Buy a good scope......check that the impact points are spot on. On the extream off chance that you have a bad scope, you can have it fixed or replaced.

Dots and hashmarks are awsome and fast......and I use them alot....but for extreme long range, lasers and Dial-ups is the only way to fly. Dots and hashmarks leave to much "uncovered" area....the area between the dots. And those areas get further apart when things get into four digit land.

Again....just my opinion.....for what ever that is worth.

 

[Pdvdh]

I don't have any problems using dots and hash marks out to 1000 yds with my holdovers. Particularly with my 3-18X IORs with the MP-8 reticles. They have 1/2 mil hash marks and a total of 10 mil holdover ability, and at 18X, the 1/2 mils equate to 1" per 100 yds of range. With the 1/2 mil hash marks, it's simple to get within 0.2 mil accuracy solely off the reticle. That equates to my holdover being within 4 inches @ 1000 yds. [0.2 mil X 2"/mill-(100 yds) x 1000 yds] = 4 inches. And I believe I can actually hold even closer than the above calculated +/- 4 inches.

This way I don't have to trust the repeatability of the turret adjustments, I don't have to worry about proper scope tracking, and I find it faster than turning turrets.

FYI: It was too windy this weekend for me to shoot 1000 yds. Once I do I'll share whatever spindrift I observe out of my 300 WM with the 210 Berger VLDs.

 

[davewilson]

jwp475 Dave, what scope did you have that tracked in a canted plane.

wasn't mine, saw it happen at the range a few years ago. guy was bitchin and moanin about everything. he needed adjustable mounts and shims cause we could barely get his gun to hit on target. he was at the end of the turret travel and when adjusting up, the POI was also going to the left. i was trying to pursuade him to get different mounts but he was hell bent on the fact that it was bore-sighted and shouldn't have this problem. if i remember right, he was "pretty close" with both turrets cranked to the end of their travel. it was a Simmons scope.

4X, I agree with you 100%. the dots/extra crosshairs are for close to intermediate ranges and work very well with a hunting setup. i'm in the process of building a carry gun that i can be effective with out to 800. a least that's the goal. i plan on using dots. somewhere in this ballpark range and certainly farther cranking turrets is the only way to go.

 

[Eaglet]

Two different rifles displaying the same pattern of behavior.

1) 300 Win Mag, 210 Berger VLDs @ MV 2910 fps. Lilja 25.5" barrel. The rifle shoots less than 0.5 moa - no problems - both with this load and 200 gr Nosler ABs.

In order to obtain a left-right zero at 1000 yds, I have to adjust the scope such that the bullets strike 4 3/4" left at 300 yds.

2) 7mm Rem Mag Tikka T3, 168 Berger VLDs @ MV 3050 fps. Tikka factory barrel. This rifle also shoots less than 0.5 moa.

In order to obtain a left-right zero at 1000 yds, I have to adjust the scope such that the point of impact at 300 yds is 7.5" left.
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Anyone have an explanation for this? My bullets are drifting left to right. Spin drift? Voodoo? I don't turn target turrets for vertical or horizontal corrections. I use mil-dot reticles for holdover. So it doesn't have anything to do with turret twisting. I've read a little about spin drift and the Correalis? effect but didn't expect either of those, even combined, to account for this. Anyone have any ideas that might explain this?

I shot both rifles yesterday in cold, dead still, conditions in order to verify what I've been observing the last three times out, and there are now no doubts about it. My bullets walk from left to right.

Phorwath,

I have not read all the posts here, but I wanted to tell you that I read an article on "Gyroscopic drift" which has nothing to do with the earth rotation, and this article said that the gyroscopic drift, on small arms ballistics, it is typical 8" to 9" drift to the right at 1000 yards for a right hand twist barrel. To that we add the coriolis efect and it looks like that might be your problem...

 

[Pdvdh]

Re: Left @ 300 yds - Right @ 1000 yds?

Eaglet,

Thanks for your post. The problem on my 7mm RM was primarily that the scope was mounted pointing off to the right rather than parallel with the barreled action. I loosened up the two piece base and shifted the picatanny bases as much as possible in the correcting directions and then loctited them back down. After that the scope was just about in perfect left-right alignment with the barreled action and I got rid of almost all of the left to right drift from 300 to 1000 yds. This little remaining drift I have could very well be gyroscopic drift. Anyhow, that rifle setup has been corrected.

As soon as I get a windless day when I can shoot my 300 WM at 1000 yds, I'll find out how much drift remains with that rifle. I purchased a US Optics bubble level that mounts to the one-piece picatanny scope base and I was surprised how much scope cant I was causing. I re-mounted that scope to ensure the crosshairs are perpendicular to the receiver based on the use of a bubble level on the scope base and a plumb bob set up at 50 yds. Now that I've got the scope reticle mounted dead nuts on, I've re-zeroed the rifle at 300 yds, and I'm just waiting for wind-free conditions to follow-up with a 1000 yd shooting to see if I'm still getting any substantial left to right drift.

I'm learning that stuff that doesn't really matter at 600 yds can be pretty important at 1000 yds. My 7mm RM with the crooked scope mounting was walking about 20 inches to the right out at 1000 yds. But I was dumb and happy shooting out to 600 yds. It didn't really show up until I extended the ranges out to no-man's land.

Think I need one of those barn burner Allen magnums that gets out there so fast that there's not time for gyroscopic drift. I hear they also cause time to stand still so there's no coriolis effects! Some of that Star Trek stuff! Well... maybe.

 

[Eaglet]

I hear you! I'm glad all is coming together.