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

[kocoa88]

Make sure that your rings are mounted true and are not putting any torque on your scope. I've seen this happen with scopes in the past causing the same kind of impacts that you are getting. A small amount of torque on the scope tube can and will give you these kinds of problems.

 

[ven]

This is a very intresting thread my rifle hits a half inch left at 350 and 3/4 to an inch left at 500 could this be down to spin drift.

I am shooting 155 smk out of a 308.I am dead on at 100 yards .

 

[Pdvdh]

Most rifles have right-hand twist rifling, which causes spindrift to occur towards the right.

You could only have spindrift towards the left if the rifling in your barrel is left-hand twist.

Lastly, spindrift doesn't become very pronounced until shooting past 500-600 yds. The farther the distance you shoot, the greater the spindrift for each additional 100 yds of range covered. Myself, I never saw it showing up with my rifles out to 600 yds in the past. It's there at 600 yds - just not enough to be concerned about unless you're trying to hit houseflies first shot at that distance. It didn't become obvious to me until shooting 1000 yards.

There would be much more spindrift from 1000 to 1500 yds than from 500 to 1000 yds.

 

[Pdvdh]

No more Left @ 300 yds - Right @ 1000 yds?

OK, finally had a cold, windless, Alaskan day today, which allowed me to shoot at 1000 yds in ideal conditions.

Problem was: My scope wasn't mounted exactly parallel to the receiver/barrel on my Tikka T3 7mm Rem Mag. It was pointed off towards the right.

I loosened up both Warne bases on the receiver and shoved them in the direction needed to get the scope parallel to the barreled receiver and then Loctited & tightened them back down to the receiver. There was just enough play in the two-piece bases to get the scope lined up properly. I also installed a bubble level on my Zeiss Conquest scope tube. I purchased an offset model from Sinclair Intl. which cost about $28. After carefully mounting the scope in a manner to ensure the crosshairs were mounted perpendicular to the receiver, I then sighted the rifle in at 300 yds last weekend.

This morning, with no wind, I shot at 1000 yds and adjusted the windage until I was hitting about 1 inch left. Next I shot the rifle at 300 yds and I was hitting about 3/4 inches left. So the impacts at 1000 yds are just a tad to the right of where they would be expected (~2.5" left) with the 3/4 inch left impacts at 300 yds. There may be a little bit of spindrift effect, but by far and away my left to right drift problem was corrected by getting the scope mounted straight with the barreled receiver. I would be stretching it to try to put a measurement on my spindrift based on the rather limited number of shots I fired this last shooting session. I only shot 3 shots at 1000 yds and 2 at 300 yds today. I would want to shoot a few more at both distances before attempting to quantify my spindrift.

In case others are interested, the BC of the Berger 168 gr VLD in my Tikka T3 7mm Rem Mag - based on the 300 yd and 1000 yd impacts - was determined to be 0.669, which is a little higher than Berger's advertised BC of 0.643. This is based on the points of impact at these two ranges - not velocity measurements at both distances.

And in my 300 Win Mag, the Berger 210 gr VLD field performance worked out to a BC of 0.675, which is also somewhat better than Berger's advertised BC of 0.631

Thanks to everyone for providing assistance in helping me get this scope mounting problem corrected. I'm feeling better now.

 

[royinidaho]

phorwath,

I've been following this thread from the beginning.

You last post pretty much nails it.

I recall posting that I though I had 6 or 7 inches spin drift from 200 to 1K. I was most certainly wrong. JBM calculations would pretty much agree with your final results. This also brings things in alignment with jwp475's post.

What if a fella experienced spin drift enough to be confident of exactly how much there actually was? What if that fella then simply rotated the scope tube level until hits were spot on @ 1K w/a 100/200 or 300 yard zero.

The idea seems a little hokey but is much simpler than davewilson's rotating the scope. Simpler may not be better, in this case?????

PS: Which reticle do you have in your conquest?

 

[Pdvdh]

I have the 4.5-14X 44mm Zeiss Conquest with mil-dot reticle and the target turrets. I consider it to be a lot of scope for the money, and wouldn't hesitate to get another. The biggest improvement in my mind would be a mil-hash reticle with hash spacings of 1/2 mil like the IOR MP-8 reticle, or some of the NF reticles. But the Zeiss mil-dot is still a very functional reticle, which I use for hold-overs and hold-unders, versus cranking the turrets. My type of hunting involves backpacking and a fair amount of travel. There aren't very many roads/trails in Alaska. The Zeiss is light compared to the NF, IOR, and other tactical scopes, which makes them nice for a carry gun.

From what I've read and researched, you could expect anywhere from 1" to 11" of spindrift at 1000 yds and it really depends on the design of the bullet. With Bergers, Nosler ABs, Sierra MKs, and the majority of manufactured bullets, I read that spindrift would likely be in the upper part of that range - say from 5-10 inches at 1000 yds - unless you find and purchase bullets that are specifically designed to defeat the effects of spindrift.

I will mention that my scope is turned ever so slightly clockwise from exactly perpendicular to the receiver. I left it mounted that way so as to provide a small level of compensation for spindrift. My bubble level is leveled up with the crosshairs also, rather than with the top of my receiver. So when I level up my Sinclair bubble, the rifle will be canted left very slightly. My goal was to counter the effects of spindrift. Perhaps that's why I wasn't seeing more obvious spindrift today after straightening up my scope mount on the receiver.

By far and away my L>R bullet drift problem was caused by a scope mounted such that it was pointed off to the right of the barreled receiver.

 

[royinidaho]

phorwath,

Thanks for the reply.

I have a conquest 6.5-20. I enjoy it for the reasons that you like yours. Mine is on a dedicated LRH, read too heavy for a mtn carry gun (but I do it anyway) in 270 AM.

One of these days we'll get a cold windless day here and I'll give it a go. I also have to wait for the ponds to freeze over so I can walk in a straight line to the 1100 yd target.

 

[Pdvdh]

Re: Article on Spin Drift

Following is a link to a site that gives a good, yet brief, explanation of spindrift. This author calls it gyroscopic drift. He says on average to count on 8-9 inches of spindrift at 1000 yds. Right twist rifling will cause rightward drift, and most everyone has right twist rifling.

Homepage of Bryan Litz - A Bravenet.com Hosted Site

"Having a left or right twist will change the direction of gyroscopic drift. Bullets fired from right twist barrels drift to the right, and vice versa by the same amount, typically 8-9 inches at 1000 yards for small arms trajectories."

 

[jwp475]

Re: Article on Spin Drift

Following is a link to a site that gives a good, yet brief, explanation of spindrift. This author calls it gyroscopic drift. He says on average to count on 8-9 inches of spindrift at 1000 yds. Right twist rifling will cause rightward drift, and most everyone has right twist rifling.

Homepage of Bryan Litz - A Bravenet.com Hosted Site

"Having a left or right twist will change the direction of gyroscopic drift. Bullets fired from right twist barrels drift to the right, and vice versa by the same amount, typically 8-9 inches at 1000 yards for small arms trajectories."

I shot 180 grain TSX & AccuBonds out of my 300 WIn and 300 SMKs out of my 338 Lapua. I do not adjust for spin drift and I have no (zero) problms hitting my targets at 1000 yards. My steel target that I shot at 1k are not more that 12", so if I adjusted for 8 to 9 inches of drift then I would completely miss my target.

I had a conversation about this with Gerald Perry the maker of the ExBall ballistics targeting software and he stated that all bullet do not exhibit enough spin drift to be concerned about. His suggestion is to shoot an a calm morning with no wind and to measure from the center of your group back to your aim point and that is your spin drift and it can be entered into the ExBall program and then the program will compensate for spin drift. He did say that long high BC VLD bullets were famous for spin drift

 

[Pdvdh]

Re: Spindrift @ 1000 yds

That's basically what I did with my 7mm RM last weekend in windless conditions. I first fired three shots into 0.36 moa at 1000 yds hitting about 1.5" left of POA.

Then I fired 2 shots @ 300 yds which hit about 5/8" left of POA.

I did not see evidence of 8-9" spindrift, otherwise my 1000 yd hits should have been to the right of my POA. This was with Berger 168 gr VLDs. But I could have had ~3-4 inches of spindrift. Not so much that I'm too concerned about it. I think I'll just let my rifle hit a little left at 3-400 yds by zeroing dead on for 1000 yds. Then call it good for my 7mm RM since I won't shoot it much farther than 1000 yds anyhow.

I'll take my 300 WM out again and shoot 210 Berger VLDs the next chance with wind-free conditions and see what I come up with at both 1000 and 300 yds. I've installed bubble levels on both rifles now to help minimize any rifle canting errors on my part.

Interesting that you're not experiencing notable wind drift with any of those three bullets. I'll report back with any spindrift info. from my 300 WM after I shoot it again.

 

[royinidaho]

Finally got a windless day. In fact, three of them consecutively. How often does that happen in Idaho? Got to shoot on the last one.

Shooting 277 140 NABs @ 359989 RPM @ muzzle with ~330000 @ 773 yards.

200 yd zero is spot on the POA.

773 yard POI was 3/4 MOA to the right (~5.8"). This was with zero/zip/nada wind. The group size was ~0.3 MOA.

The target is now sitting out at 1007 yds awaiting another windless day'

Holey crap, I read that article.

My next question is does Exbal or any other ballistics software automatically compensate for the two subject drifts.

Or do I alway want to sight in and hunt up in Lemhi Idaho which is pretty much half way between the equator and north pole. Then, do I want to always shoot north? Or is it south? I think I'm getting the vapors.

Also I think it may be better to have a rifle that shoots for crap and limit the distance to 300 yards A fella could cut way back on the "nice" pills.

 

[MagMan]

Roy a muzzle velocity of 3499.893056 fps is pretty good but are you sure it wasn't 360000 rpm as that would be an even 3500 fps.

Then again I too hate those even numbers.

 

[royinidaho]

****, you had me there for a minute.

The muzzle number was directly from the RSI print out. The print out shows only 750 and 800 so I guestimated the 773 value and rounded off a bit. Sorry about that. The actual number is: approximately 330,499.58000000002 according to my mac mini calculator.

BTW, I take pills for this condition but apparently not enough of 'em

 

[jmason]

My next question is does Exbal or any other ballistics software automatically compensate for the two subject drifts.

I believe "Field Firing Solutions" does. That is if your talking spin drift and earths rotation.