20 MOA Base Problems

[txhoghunter73]

I have a 20 moa base on my 300 RUM sendero with a Vortex PST 6-24x50. I have sighted it in and done my load work up. Approximately 125 rounds through it all done off a bench at home. So today I get in the deer stand and I stick the rifle out the window to make some adjustments to my power and pa and I notice I have a horrible white haze in the scope. So I start moving my head around to see if I have an eye relief issue and I notice that if I lift my head up the haze goes away and the scope becomes clear, but now the top portion of my reticle is cut off. It's like the scope has too much forward cant for this shooting position. Anyone had this issue? If so, what was your fix?

 

[txhoghunter73]

Up^

 

[txhoghunter73]

^ttt

 

[Barrelnut]

I have basically the exact same setup and don't have this issue. Seems you would have noticed this at the bench during load workup. Are you sure it's not the frame of the window the gun is sticking out of that you're seeing?

 

[txhoghunter73]

I have basically the exact same setup and don't have this issue. Seems you would have noticed this at the bench during load workup. Are you sure it's not the frame of the window the gun is sticking out of that you're seeing?

Yessir. And I checked both lenses.

 

[dok7mm]

I would check parallax setting and eye relieve ( due to different cheek weld vs shooting off bench). I don't think it has anything to do with a 20 moa base if everything is still tight. What is it like after you left blind?

 

[FEENIX]

I would check parallax setting and eye relieve ( due to different cheek weld vs shooting off bench). I don't think it has anything to do with a 20 moa base if everything is still tight. What is it like after you left blind?

+1! I hope it is not the case, but it could be an internal damage. A friend had a similar experience with his .340 WBTY (can't remember the scope brand/model but I know it's on the low end, IIRC it's a Bushnell ???) while bear hunting. Anyways, he went in the wooded side and he came out about an hour later; a quadrant of his view is hazy/internal fogging. Ended hunt early.

He sent the scope out for repair and apparently there was a small internal leak. IIRC, they re-seal and re-purge it. He ended up replacing it with a Leupold scope.

I hope you have it resolved for your hunting season. Good luck!

 

[txhoghunter73]

+1! I hope it is not the case, but it could be an internal damage. A friend had a similar experience with his .340 WBTY (can't remember the scope brand/model but I know it's on the low end, IIRC it's a Bushnell ???) while bear hunting. Anyways, he went in the wooded side and he came out about an hour later; a quadrant of his view is hazy/internal fogging. Ended hunt early.

He sent the scope out for repair and apparently there was a small internal leak. IIRC, they re-seal and re-purge it. He ended up replacing it with a Leupold scope.

I hope you have it resolved for your hunting season. Good luck!

I really hope that's not the issue. I don't think it is though it's more like I'm have a hard time getting my head square with the eye peice, because when I can see the whole objective it's foggy but when I lift my head up and kinda look down at it it becomes very clear but it cuts a small amount of the reticle off. I took it down to my shooting bench and put it on the bags and I have neither of the issues, but I think that's because I'm shooting on a 5 degree incline instead of a decline like I would be when I hunt from the box blind. I'm beginning to wonder if I need to ditch the 20MOA rail, but I really don't want to.

 

[WapitiBob]

You should be able to get behind the scope, set your head of full scope eye relief then move your head l/r and up/down and lose the full scope slightly with each movement. If you're seeing fog vs the black I would say your scope is hosed up.

 

[txhoghunter73]

You should be able to get behind the scope, set your head of full scope eye relief then move your head l/r and up/down and lose the full scope slightly with each movement. If you're seeing fog vs the black I would say your scope is hosed up.

Hosed up?

 

[txhoghunter73]

I called vortex, he said they here about this issue all the time when people put a 20 moa base on a hot rifle, he said that it has to do with too much negative adjustment on the reticle. Does this seem right to yall?

 

[westcliffe01]

That scope has 65MOA of elevation travel. So half that is 32.5 MOA. With a 20MOA base you would have had to dial down 20MOA to get on target, leaving 12.5MOA give or take a little based on the rail and receiver tolerance.

You should be operating the scope will within its design specification. Any substantial target image degradation should be cause for a warranty repair.

But it is easy to test. Dial the scope UP 10 MOA and see what the image quality looks like ? Then dial it up another 10MOA and try that.

My personal recommendation is to not use that scope above 20x. So just dial it down a little and see if that resolves the issue. The exit pupil is very small at 24x and your eye position has to be just right to get a proper image. The difference between 18x 20x and 24x should not be a significant factor in taking a shot at pretty much any reasonable distance.

 

[txhoghunter73]

That scope has 65MOA of elevation travel. So half that is 32.5 MOA. With a 20MOA base you would have had to dial down 20MOA to get on target, leaving 12.5MOA give or take a little based on the rail and receiver tolerance.

You should be operating the scope will within its design specification. Any substantial target image degradation should be cause for a warranty repair.

But it is easy to test. Dial the scope UP 10 MOA and see what the image quality looks like ? Then dial it up another 10MOA and try that.

My personal recommendation is to not use that scope above 20x. So just dial it down a little and see if that resolves the issue. The exit pupil is very small at 24x and your eye position has to be just right to get a proper image. The difference between 18x 20x and 24x should not be a significant factor in taking a shot at pretty much any reasonable distance.

I agree. I don't think I even need a 20 moa rail to get this rifle out to 1k, so I may just ditch it.

 

[FEENIX]

I agree. I don't think I even need a 20 moa rail to get this rifle out to 1k, so I may just ditch it.

This is probably the best advice you'll ever get from an optics SME ...

Even though the scope adjustment range may be large enough to get to 1,000 yds, you should use a 20 moa base. That's because off-axis optical aberrations that degrade resolution increase with incidence angle. You should set up your rifle so that the incidence angle is minimized for long distance shots (where resolution matters most).

Assume the base is within +/-10 moa of alignment with the rifle bore. Let's also assume your bullet drops no more than 30 moa at 1,000 yds, and you like to zero your rifle at 100 yds. With a standard base, you would need up to -40 moa of adjustment to get to 1,000 yds. That means you need a total 80 moa of adjustment.

With a 20 moa base you would need up to -20 moa to get to 1,000 yds, but up to 26 moa to get to a 100 yd zero (worse case boresight alignment in each case). That means you need a total 52 moa of adjustment. With a 20 moa base, however, the incidence angle at 1,000 yds is 20 moa less, so the image will have less blur.