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Scope runs out of adjustment
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I finally got to the range to sight the 375 RUM w/350 gr SMKs in at 300yards and install the bullet drop comp dial on the Leupold VX6 4-24x. the scope is mounted on a M70 with dual Dovetail rings, parallel to the bore. Things went well until I went to shoot at farther ranges, The scope ran out of adjustment at 425 yards. The dial is calibrated to 1050 yards. Talking to the tech at Leupold he said the solution is to change out the mounts to a picatinny style that has 40 MOA slope built into it. Leupold makes such a style base/ring but they only tilt it to 20 MOA, so I need to find the 40 tilt somewhere else.


I can understand the scope running out of adjustment, but to do so at just 425 yards seems a little shortsighted in scope design.


Is this normal or is it possible the scope has a problem?
 
Posts: 1421 | Location: WA St, USA | Registered: 28 August 2016Reply With QuoteReport This Post
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Ray, I would say it is your rifle or perhaps your mounts. Are you using two piece mounts? Is the correct one in the front and back?

I have a Sendero that is deadly accurate but it runs out of adjustment around 1000 yards with a Nightforce NXS. I have a standard rail but need to replace it with a 20 MOA rail.

Really dumb question: your set screws on the elevation are tight right? I don't know about VX6 but Leupold VX3s set screws always seem to loosen on their own; you can tell when it doesn't click, so I like I said, I doubt it is that. If you have an optical bore sighter, stick it in and run the dial up and down to see if it is moving. Remember, as you click up in elevation the crosshair will move down on the grid.


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Posts: 7253 | Location: Arizona | Registered: 28 July 2004Reply With QuoteReport This Post
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Above are pics of the mentioned rifle and scope. Everything is set-up according to mfg specs and all screws are tight.
 
Posts: 1421 | Location: WA St, USA | Registered: 28 August 2016Reply With QuoteReport This Post
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Ray, that's not uncommon when shooting long distance. My guess is your 350gr load doesn't shoot as flat as most "long range" cartridges/loads do. 20 MOA is a very common size, 40 MOA is uncommon.

I would be surprised if 20 MOA wasn't enough to do what you need to at 600 yards or a little more. If not you can get a one piece 20 MOA base and shim the -front- rear . I've used shims cut from soda cans but you can buy better, factory made shims of various thicknesses. Don't try that with 2-piece bases or ring alignment will be off. Alternatively, you can try shooting a lighter bullet at higher velocity.

Here's something to consider. The bullet drop dial on the scope was made for a specific load and either a standard or angled base. With a different load and angle of base the yardage increments on the dial will be off, slightly at some distances and way off at others. Your best bet would be to use the load the scope is calibrated for. If not, you can pretty much ignore the yardage marks. You can also contact Leupold and order a set of dials for your 350gr load, specifying the 20 MOA base. If that won't adjust to out 1000 yards (I'd be surprised if it did} then you will have to use it at whatever the maximum distance is and closer.




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Posts: 10510 | Location: North of the Columbia | Registered: 28 April 2008Reply With QuoteReport This Post
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Here is one other thing to think about: are you relatively centered in terms of windage? The more you crank left or right, the less vertical adjustment you will have because the erector tube will "hit" the scope tube sooner.

I am not big on shimming; it works, but the base is not bearing on the receiver - just the shim. But if you do shim, put it under the rear portion, not the front.


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Posts: 7253 | Location: Arizona | Registered: 28 July 2004Reply With QuoteReport This Post
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quote:
Originally posted by AnotherAZWriter:
... if you do shim, put it under the rear portion, not the front.
Absolutely. What was I thinking? Corrected above.




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Posts: 10510 | Location: North of the Columbia | Registered: 28 April 2008Reply With QuoteReport This Post
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quote:
Originally posted by Grenadier:
Ray, that's not uncommon when shooting long distance. My guess is your 350gr load doesn't shoot as flat as most "long range" cartridges/loads do., you can try shooting a lighter bullet at higher velocity.




For comparison using Sierra data, a 6.5 Creedmoor has MV 2700 fps with a few more with the 150 gr SMK, BC above 1760 fps .713. The 375 RUM has MV 2600 fps with the 350 gr SMK BC above 2200 fps .805. I don't have a bullet drop program handy but seems to me that 2700 fps w/BC.713 will be quite similar over 1,000 yards to 2600 fps w/BC .805

The dial was made by Leupold for the VX6 4-24x scope with parallel mount on the 375 RUM w/350 gr SMK with MV 2600fps Switching to a lighter bullet to gain MV would be offset by reduced BC and while gaining a flatter trajectory in shorter yardages would be lost at longer distances.

So in terms of gain for 20 and 40 MOA, how much slope is involved with the mount. In other words, how much higher is the rear portion of the base than the front? I'm aware that there would be problems with the rings crimping the scope to just elevate the rear ring, but I'm wondering what the rail would look like.
 
Posts: 1421 | Location: WA St, USA | Registered: 28 August 2016Reply With QuoteReport This Post
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I'm not sure at what distance they measure the MOA amount you lift the scope at the rear with the new base.
probably 100 yds, if so that's well over 20"s of change.
 
Posts: 2965 | Location: soda springs,id | Registered: 02 April 2008Reply With QuoteReport This Post
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quote:
Originally posted by Ray B:
quote:
Originally posted by Grenadier:
Ray, that's not uncommon when shooting long distance. My guess is your 350gr load doesn't shoot as flat as most "long range" cartridges/loads do., you can try shooting a lighter bullet at higher velocity.




For comparison using Sierra data, a 6.5 Creedmoor has MV 2700 fps with a few more with the 150 gr SMK, BC above 1760 fps .713. The 375 RUM has MV 2600 fps with the 350 gr SMK BC above 2200 fps .805. I don't have a bullet drop program handy but seems to me that 2700 fps w/BC.713 will be quite similar over 1,000 yards to 2600 fps w/BC .805

The dial was made by Leupold for the VX6 4-24x scope with parallel mount on the 375 RUM w/350 gr SMK with MV 2600fps Switching to a lighter bullet to gain MV would be offset by reduced BC and while gaining a flatter trajectory in shorter yardages would be lost at longer distances.

So in terms of gain for 20 and 40 MOA, how much slope is involved with the mount. In other words, how much higher is the rear portion of the base than the front? I'm aware that there would be problems with the rings crimping the scope to just elevate the rear ring, but I'm wondering what the rail would look like.


You can solve it using similar triangles; assuming the screw spacing on the rail is 6", then you have X/6=20/3600, which means 6x20=3600X. Solving for X, we get .033, or 33 thousands of an inch, which is difficult to see with your naked eye.

Note: 20 is 20 inches which is 20 MOA at 100 yards. 3600 is the number of inches in 100 yards.

In terms of "crimping" the scope or rings, the bases have two flat surfaces, so you are not inducing stress. You are inducing stress however, if you use a shim.


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Posts: 7253 | Location: Arizona | Registered: 28 July 2004Reply With QuoteReport This Post
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Leupold specs indicate your scope has 68 MOA of adjustment, supposedly 34 up and 34 down from adjustment center. But things don't always work out that way and you may well have less than 34 MOA of upward adjustment. Assuming you do have 34 MOA, that would barely be enough for your load at 1000 yards (I figured 33.83 MOA @ STD ATM) with nothing to spare for terrain or environmental differences.

The easy solution is to use a 20 MOA 1-piece base. 20 MOA should do it for you. If not, then you can shim it. Shimming does not crimp the scope or rings if you use a 1-piece base.




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Posts: 10510 | Location: North of the Columbia | Registered: 28 April 2008Reply With QuoteReport This Post
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Like I said above, make sure your windage is relatively centered. If you are way to one side (for whatever reason) you will not have 34 MOA up.


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Posts: 7253 | Location: Arizona | Registered: 28 July 2004Reply With QuoteReport This Post
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Set your cross-hair in the middle of its range; sounds like 34 up from the bottom. Bore sight on an object. If your cross-hair is on the object as well, the height of your bases is appropriate.
 
Posts: 135 | Registered: 02 December 2005Reply With QuoteReport This Post
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Get you some Burris Dual Dovetail SIGNATURE rings. Looks like a 30MM scope tube. The 30MM SIGNATURE rings come with +10 and -10 inserts. You can gain 20MOA with just the set of rings. The Dual Dovetail rings will fit the Leupold bases. I think Burris quit making them in Dual Dovetail but you might find some.

God Bless, Louis
 
Posts: 1242 | Location: Mountains of North Carolina | Registered: 14 January 2008Reply With QuoteReport This Post
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the scope is 34mm, but I'll check with Burris. Thanks for the tip.
 
Posts: 1421 | Location: WA St, USA | Registered: 28 August 2016Reply With QuoteReport This Post
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What I found out the hard way. Picatinney rails are much taller than standard type bases. You will probably need low or extra low rings especially if you are shooting prone.


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Posts: 2086 | Location: Houston, TX. | Registered: 18 May 2004Reply With QuoteReport This Post
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