Scope mounting-centering left and right- I need some experienced input

[Coaster]

I have an older Rem 700 sps tactical in .308, 20 inch barrel, Zeiss Conquest with zplex 1000 reticle. It's zeroed at 100 yards for test grouping ammo I reload. I just used a digital chronograph and found my loads are a little slower than the manual suggests...most likely my short barrel...2450 as opposed to the 2600 I was loading for. Two questions: I'm dead on at 100 yds, dead on at 50 yds (I know it's close, but I shot at my .22 target to get my chronograph readings) but my 4 inch group at 500 yds was hitting 14 inches right...this with a 5 mph tailwind 5 degrees left!? Is this spin drift? Is it possible that my scope needs shimmed left or right...not sure if it's zeroed close if it should also be zeroed out that far or it's canted slightly and needs shimmed. Also, I have a Seekins 20 moa rail mounted...if I had a flat rail would it make my reticle range marks match better or does it matter? I know the ammo velocity should be around 2700 fps which will obviously help it match the 500 yard mark on the reticle as opposed to the 2450 I was getting with this test (it was hitting down at the 600 yard mark). The scope is mounted nice and low to the barrel, maybe 1/8 or 1/4 inch clearance. The rifle groups great...I think I can get smaller than 4 inch grouping at 500 yards with better weather conditions, but the right drift at distance and matching my yardage marks in my zplex 1000 reticle are my main objectives. So, do I need to shim the scope left/right or is it another issue? And will a flat rail as opposed to my 20 moa rail help match my yardage marks in my zplex 1000 reticle? I haven't drawn it out on paper yet to understand what is happening...thanks for any input, this forum is awesome!

 

[shovelstrokeed]

Range marks on the reticle rarely match up to paper or steel unless you are using the exact bullet/MV/weather conditions the reticle was calibrated for.
No, the cant of the mount is not gonna matter either.
It sounds lie you need to true your rail. Most any scope will do this if the rail is crooked. Center line of the rail has to match bbl center line. Sounds like you need the services of a good gunsmith to remount your rail.

 

[brianf]

reticles with yardage marks are a great idea for quicker shots, but if you are looking for down to the nats ass accuracy you will either have to dial the elevation or calculate the proper hold. the ret will get you close and maybe even spot on at times but diff ammo, temp and such will play havoc through out the year.

the actual scope mount/rail (as long as it is tight, and inline with you barrel and action) should have nothing to do with the 14". spin drift only start to really be notice a loong way out.

you seem to have quality parts so i do not think you need shims at all.

what it seems like is that your scope is canted on the rifle. hang a string or take a level and sharpie marker to the range and see how your scope lines up. also you might want to try a bubble level, a little bit of cant makes a big difference.

 

[Coaster]

Thanks Shovel and Brian! That is all I needed to know and you guys are a big help! I'll work it out now.

 

[Graywolf.260]

Before you mess around with your rail or cant check your parallax adjustment (if it's a 4.5-14). If it's a 3.5-10 there is no parallax adjustment on the Zeiss and you need to be very careful with how you are lining up behind the rifle and scope.

 

[Coaster]

Roger that wolf, I'll be consistent with my positioning and holds for parallax, it is a 4.5x14. The scope can't is not an issue, it's mounted using a rifle rail and scope level system. I did some math and found that if the rail is off it's sitting roughly .003" to the left. Trigonometry, guess I did use it hah! I'll do some rigged testing before I get any machining done. Thanks again guys, awesome!

 

[nfoley]

If it were me I would want to verify it did this on at least two different trips to the range before messing with the rail. Verify you have no cant with a plumb line (not sure what system you used but those wheeler level-level systems are not that great). Could be wind was doing something squirrely down range, or possibly you were doing something different with regards to your form (no offense - just throwing that out) with the 500 yd group.

 

[Coaster]

Yes, planning a second session to verify the problem...I do know where to hold to hit the center, 600 yd line at the 5mph right wind age mark heh! Thanks for the input guys, I'll post again when I get it fixed and let you know what was going on.