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

[Pdvdh]

I looked over my scope set up over last night very carefully. The scope tube isn't rotated counterclockwise. If anything it leans perhaps a tad clockwise. I'm using TPS rings on my 7mm, and it did look like the scope tube alignment left-right was slightly off when looking down along the receiver & barrel. Thanks for posting. I'll need to check out my left-right scope tube alignment more carefully tonight.

 

[Pdvdh]

I got ahold of some small levels and hung a plumb bobb at about 50 yds. The 7mm Rem Mag scope reticles are mounted with just a slight clockwise cant on the receiver, but this should have directed the bullets to drift left if I leveled off the reticles - so this couldn't be causing the left to right drift.

However, based on Jim5351's post, I took a close look to see if the scope was mounted parallel to the receiver and barrel by viewing the scope from above the receiver, and sure enough, the scope is mounted - windage wise - pointing slightly off to the right. I think Jim 5351 may have identified the problem. I'm using two-piece Warne bases, and I'm in the process of loosening each base, pushing them in the direction required to help straighten the scope out - windage wise. I won't be able to gain a lot of correction doing this but I will gain some improvement, and maybe enough. One of my scope base screws broke so I'll have wait till tomorrow to get the scope put back on the rifle. Then I'll head back out & re-zero at 300 yds, followed up with some 1000 yd shots to see if I made any headway.

If anyone has read of this type of scope mounting defect causing left to right, or right to left, bullet drift at long distances, could they post a link to an online source here? I don't know as much about this as I'd like to. But it makes sense that if the barrel is pointing to the left of where the scope is pointed, that the bullet would have to be directed towards the right to intersect with the crosshairs at any given sight-in range. Then once the bullet crosses the sight-in range, it should continue to drift to the right slightly at all ranges past the sight-in distance.

Some windy weather has moved in here now. I'll post again with my results after the wind dies down and I get to shoot at 1000 yd again.

 

[davewilson]

phorwath said, The 7mm Rem Mag scope reticles are mounted with just a slight clockwise cant on the receiver, but this should have directed the bullets to drift left

i would disagree with this statement. if you're talking about clockwise in the direction that we look through the scope, this would cause the bullets to impact to the right. this is what you stated you have with both setups.

when i set my scope, i actually have it i bit counterclockwise. when shooting at 100 yards and up 30MOA, i like the bullets to impact .5-.75 to the left of verticle. with this setup, it usually hits a bit left at 5-600 yards, but it's straight at 1000. this is how i deal with spin-drift.

 

[Pdvdh]

davewilson,

I wonder if we're meaning the same thing but saying it differently. Let me describe my scope reticle condition one more time and then see if we agree or disagree. Clockwise and counterclockwise as used in this post are always from a point of reference behind the rifle/scope.

My 7mm RM is a Tikka T3 Lite with a flat section of receiver on the top of the receiver. So I took a small level and supported the rifle until the level showed that the top of the receiver was level with respect to being rotated in either the clockwise direction or the counterclockwise direction. Then I strung a plumb bobb about 50 yds away and placed the vertical crosshairs right along the plumb bobb string so I could see if the crosshairs were straight up and down with the string. Looking through my scope (with the top of the rifle receiver leveled), my crosshairs were rotated ever so slightly in the clockwise direction from the plumb bobb string. The very top of my vertical crosshair was offset to the right of the string while the very bottom of my vertical cross hair was slightly offset to the left of the string.

Now with the scope/crosshairs mounted on the receiver in this slightly clockwise from plumb with the receiver position, in order to place the vertical cross hair in perfect alignment with a plumb bobb string, I have to rotate the rifle very slightly counterclockwise (again - viewed from the shooter's position). By rotating the rifle counterclockwise before touching off a round, the bore and bullet will be directed up, and ever so slightly towards the left in order to hit my point of zero, because by rotating the rifle counterclockwise the end of the muzzle ends up being positioned slightly off to the right hand side of the intersection of my crosshairs.

So in order to counter the effects of spindrift, one could mount the scope so the crosshairs were rotated slightly clockwise from being plumb with the receiver, and then by leveling the vertical crosshair up with force of gravity, the bullets would be sent down range with a slight right to left drift, which could counteract the left to right drift caused by spindrift.

That's how I see it and describe it. Do we agree or disagree? Let me know what you think.

 

[royinidaho]

all of this is giving me the vapors!

When all is said and done would/could someone summarize what a simple fella like myself could do to get spin drift out of clicking process.

I've never noticed it due to Idaho's windy conditions. But this may also be why my windage charts are always off!!! It just so happens that I do most or all of my shooting when wind is from the left.

I like Dave's idea of mounting the scope a bit off perpendicular, but do you have any idea of how difficult that is to get correct.

I'd rather just adjust the windage part of the drop chart to compensate but that would seem to screw up other things such as when the wind direction is coming from different directions.

Some times I feel just dumb.........but I really want that first shot to be spot on. On distance rocks I'm 80% spot on w/the second shot and 100% w/the third shot. Elevation is spot on w/first shot but it takes two more to get the windage right. W/spin drift factor'd in things would improve immensely,

Thanks fellas

 

[squirrelduster]

Royinidaho-
Set your scope so it is zeroed for theoretical spindrift at 1000.
If your theoretical spindrift at 1000 is 1 moa to the right set your scope at 100 yards to be 1 moa to the left and have that as your zero.

Unless you are shooting mice at 500 yards you should be dead on with less than 3 inches to the left. Mr. coyote or anything larger will never know the difference. This works great with drop charts.

 

[Pdvdh]

I've read that some others do what squirrelduster has suggested. That's basically what I did with my 300 WM. While shooting it at 1000 yds with "no wind conditions", I then adjusted the windage turret to the left until I was dead on at 1000 yds. Same thing. Then when I followed up and shot at 300 yds it was hitting left.

Others will rotate and mount the scope slightly clockwise from perpendicular on their receivers. Then when they level up their crosshairs the rifle is canted slightly such that the bore & bullet is directed in a leftward drift, to oppose the rightward effects of spindrift.

Others say the heck with it. They feel that wind is the overwhelming factor in their left-right misses and don't bother with it. I'm going to correct for it one way or the other, because after the fourth time shooting both my rifles at cardboard targets at 1000yds, it became clear to me that there was a definite rightward drift to my bullets. Now I know it was partially caused by spindrift. So we're learning something together here.

 

[jwp475]

nO wind that I could detect and no spin drift adjustment

 

[royinidaho]

Royinidaho-
Set your scope so it is zeroed for theoretical spindrift at 1000.
If your theoretical spindrift at 1000 is 1 moa to the right set your scope at 100 yards to be 1 moa to the left and have that as your zero.

Unless you are shooting mice at 500 yards you should be dead on with less than 3 inches to the left. Mr. coyote or anything larger will never know the difference. This works great with drop charts.

OK, next question.

How does one go about determining theoretical spindrift @ a distance. I'm am convinced that spin drift is a factor....now. As when I shoot 1100 w/a 200 yd spot on zero, on my normal range, the wind is pretty much always the same. My first two shots were spot on elevations wise and about 10" right w/a 3" spread.

Clicking to the left the appropriate amount solved all problems. I would sluppose that spin drift and wind were compensated for in that adjustment.

However when I go elsewhere and the wind id from another direction I'm in a world of hurt.

The goal is first shot 1K yote accuracy.

Bullet: 140NAB, 7 Twist, 3600 fps MV.

 

[jwp475]

Roy all bullets do not exhibit spin drift, at least enough to worry with. The VLD Bergers and some of the SMK's will. So Zero your rifle and on a calm no wind day shoot at 600 and measure the distance from the center of your group back to your aim point and them imput that into your ExBall program and it will automaticaly correct for your spin drift at all ranges.

 

[royinidaho]

Roy all bullets do not exhibit spin drift, at least enough to worry with. The VLD Bergers and some of the SMK's will. So Zero your rifle and on a calm no wind day shoot at 600 and measure the distance from the center of your group back to your aim point and them imput that into your ExBall program and it will automaticaly correct for your spin drift at all ranges.

Thanks


I just ran some numbers thru JBM not knowing what most of the inputs should be but stuck 'em in anyway. Results were about .9 MOA @1000??? Which "feels" about right.

 

[jwp475]

Roy ther is a spot in the Exbal program to imput your measured drift..

 

[davewilson]

davewilson,

I wonder if we're meaning the same thing but saying it differently. Let me describe my scope reticle condition one more time and then see if we agree or disagree. Clockwise and counterclockwise as used in this post are always from a point of reference behind the rifle/scope.

My 7mm RM is a Tikka T3 Lite with a flat section of receiver on the top of the receiver. So I took a small level and supported the rifle until the level showed that the top of the receiver was level with respect to being rotated in either the clockwise direction or the counterclockwise direction. Then I strung a plumb bobb about 50 yds away and placed the vertical crosshairs right along the plumb bobb string so I could see if the crosshairs were straight up and down with the string. Looking through my scope (with the top of the rifle receiver leveled), my crosshairs were rotated ever so slightly in the clockwise direction from the plumb bobb string. The very top of my vertical crosshair was offset to the right of the string while the very bottom of my vertical cross hair was slightly offset to the left of the string.

Now with the scope/crosshairs mounted on the receiver in this slightly clockwise from plumb with the receiver position, in order to place the vertical cross hair in perfect alignment with a plumb bobb string, I have to rotate the rifle very slightly counterclockwise (again - viewed from the shooter's position). By rotating the rifle counterclockwise before touching off a round, the bore and bullet will be directed up, and ever so slightly towards the left in order to hit my point of zero, because by rotating the rifle counterclockwise the end of the muzzle ends up being positioned slightly off to the right hand side of the intersection of my crosshairs.

So in order to counter the effects of spindrift, one could mount the scope so the crosshairs were rotated slightly clockwise from being plumb with the receiver, and then by leveling the vertical crosshair up with force of gravity, the bullets would be sent down range with a slight right to left drift, which could counteract the left to right drift caused by spindrift.

That's how I see it and describe it. Do we agree or disagree? Let me know what you think.

i understand what you're saying but not sure i agree. if you're saying you sight in with the receiver verticle, CW scope, and then when you shoot long range, you hold the crosshairs verticle. i would agree with your assessment. your scenario will only be true if you position the scope differently when sighting in and shooting long range. BUT, i'm thinking you probably sight in with the crosshairs verticle. with this scenario, i don't care if the receiver is at a 45 deg angle to the scope, the bullet is still going where it's sighted in. of course i'm exagerating to make a point. i'll apologize as i didn't fully understand your point till i reread your post. i posted a little quick.

my point is if you're shooting and the crosshairs are positioned any amount of CW orientation, the bullets will impact to the right, not counting spin drift.

my edge shooting 250SMK's drifts around .75 moa at 1k. it's pretty straight out to 600.

 

[Pdvdh]

dave,

Thanks for you're follow-up post! I appreciate the confirmation that we were thinking similarly, even though it appeared we were posting contrary positions. If the wind lets up tomorrow I may have a chance to shoot again. My 7mm RM was drifting too far to the right for spindrift alone to explain the drift so I'm eager to get the scope mounted straighter on my receiver and then shoot @1000 & 300 yds again.

**Post #7 in the following thread seems to confirm the need to get the scope left-right aligned properly when mounting it on the receiver in order to avoid the types of long range drift I'm experiencing. Funny I'd never read about the importance of this, or taken note of it, prior to now.

Check out post #7 in this thread: http://www.longrangehunting.com/forums/f37/basic-gunsmithing-you-can-do-35352/