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Rifle Scopes Scope change from hell or why can't I leave well enough alone?

DZelenka

Private
Full Member
Minuteman
Feb 4, 2011
48
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63
Louisiana
While at the NRA show, I had a chance to look at all of the nice glass there and the Vortex Viper PST F1 4-16x50 caught my eye. I decided that it needed to find a home on my 7 SAUM hunting rifle. So the saga begins.

Two years ago, I had LA Precision build a M700 LA into a 7 SAUM with a 26" #5 Krieger barrel. At the time, I did my homework and came up with Burris 30mm Zee rings on Burris 2 piece Weaver style bases to mount a Leupold 6.5-20x50 LRT scope. The rifle doesn't have an adjustable comb and I like my hunting scopes mounted as low as possible. All was right in the world.

Enter the Vortex PST. The first thing that I noticed was that it had less elevation adjustment in the scope so I decided to change the rings to Burris Signature Zee rings and use the offsets to give me some built in elevation. The next thing I noticed was that the scope tube is about 1" shorter than the Leupold bringing the objective bell almost into contact with the thicker part of the barrel (less than .010" clearance) using only a .010" offset in the rear (couldn't use the front offset at all). Not to be deterred in my objective of keeping the scope mounted as low as possible and wanting some built in elevation in the base, I went back to the drawing board.

New solution: I picked up a Warne 20 MOA rail and since I was using a higher mount, I decided to use the lowest rings I could find, the Seekins low 30mm (which are a work of art). Parts come in and I discover issue #3 - the power ring of the PST measures a very generous 1.760". In order to mount the scope far enough forward to provide proper eye relief, I had to mill about .075" from the top of the rear pad of the mount.

Now that all of this is finished, I have a set-up with which I am very happy. It looks good, functions great and fits my head position really well. But the journey illustrated that changing one little thing may not be so simple. :)
 

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I shot two 3 shot groups sighting it in. The "big" one with a called high shot was right at .4". Running the scope up 18 MOA from its 100 yard zero moves the point of impact .4" to the left at 100 yards. I am thinking this is going to be taken care of with the spin drift of the bullet. I can't wait to get to the 600 yard range.
 
It likely means that your scope is slightly canted.

I really hope your kidding with the spin drift comment.
 
Wow man! It's good you're handy figuring out things!

When you saw the shift after cranking it up, are you sure the reticle was level in both cases?
 
It likely means that your scope is slightly canted.

I really hope your kidding with the spin drift comment.

It either means that the scope is slightly canted or the scope doesn't track precisely. Don't know which yet.
 
Wow man! It's good you're handy figuring out things!

When you saw the shift after cranking it up, are you sure the reticle was level in both cases?

Reticle was lined up with the upside down T which was placed on the target board using a level. Scope was installed first by leveling the rail, then I leveled the scope. This method has worked well in the past. I can twist the scope to take out the little horizontal movement observed. Is it necessary for shooting under 1000 yards? I am not one who settles for less than right, but I have been known to get carried away from time to time.

OK. Talk to me about spin drift. Will a bullet drift in the direction of the twist over medium to long distance (600 to 1000 yards)? Does it not exist? I am talking about less than 1/2 moa at 800 yards. Virtually all of my experience with mid to long range shooting involves high power competition with open sights. A 1/4 min click or two at that distance can easily be attributed to wind.
 
I dont know much about spin drift but I do know that the guys that built M1903 Leaf sights had way too much time on their hands thinking about what they wanted that sight to do and how many different ways they could accomplish it.

In addition to the three or four sighting notches they included a spin drift into the ladder sight so that as you raise the slide for elevation/distance the sight tracks to the left to account for the drift of the bullet right.

Modern shooters may not be affected by spin drift but the manly men that carried the 1903 had it covered.
 
I took a quick look at Bryan Litz' applied ballistics book and he has a chapter on "gyroscopic drift". He says it is predictable and has a formula. I guess it exists. So, what is the deal?
 
Box test your scope and confirm all of your setup is level and square, including your base depending on how you're leveling your scope as well.