Drawbacks of a 20 MOA scope base?


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Z-Michigan
June 19, 2010, 11:06 PM
I have a FN PBR (similar to a Winchester Model 70) in .308 Win and am looking at installing a one-piece scope base that has a built in 20 MOA of elevation. I am wondering if there are any drawbacks to using such a base. Of course, I realize that the built in elevation will require me to dial back a fair bit on the elevation knob of the scope to get a 100 or 200 yard zero. To be specific, however:

-Is there any adverse effect on the scope from using it with elevation adjusted way down to one end of its adjustment range? Will this cause any optical distortion, added parallax, or any other problem in scope use?

-Will this limit my scope options? Some scopes only have 50-55 MOA of total internal adjustment. Are they normally designed with the 0 MOA point being right in the middle of that range (therefore allowing 25-27 MOA of downward adjustment and accommodating the 20 MOA base elevation), or do they typically have greater upward adjustment than downward adjustment?

-Any other drawbacks I should be aware of?

I already have the 20 MOA base, which I got for free. I could instead buy a 0 MOA base for $40 or so but would rather not unless the 20 MOA base is going to cause me problems.

I'm aware that the 20 MOA base shines at long range in allowing you to use almost the full elevation range of the scope to dial in elevation for long shots, however while I would like to try long range shooting the longest range I have available is only 300 yards, and the longest that's convenient is only 200 yards, so I won't get the benefits all that often.

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Friendly, Don't Fire!
June 20, 2010, 02:07 AM
I know a guy who put something similar to that on a Armalite 50bmg. He wanted me to sight it in for him, which I was extremely happy to assist!

Even with a really high-quality Leupold scope, worth about $1,500. - the absolute lowest I could "dial down" was about 30" high at 200 yards. I tried unsuccessfully to sight it in at 200 yards, with a huge piece of paper.

Out of total frustration after about six rounds, not having a clue where they were hitting, I finally aimed at a rock on a huge sand bank (about 100 feet high) in the background of the 200 yard range, and could easily see the bullet placement on the sand bank WAY OVER that boulder! Finally, once I could see just how high I was, I was able to hit some basketball-sized rocks in the lower part of the bank - off a bench, by holding about two feet high, with the scope as low as it would go.

Since, around here, there is really nowhere to safely shoot 500 to 1,000 yards, he ended up putting a regular base/mount on so he could have fun shooting closer range.

dakotasin
June 20, 2010, 05:55 AM
Is there any adverse effect on the scope from using it with elevation adjusted way down to one end of its adjustment range? Will this cause any optical distortion, added parallax, or any other problem in scope use?

in a decent scope, no there won't be these issues to any great degree. anytime you dial away from center these issues crop up, but the quality of your scope will determine how noticeable it is. any scope worth sticking on a 20 moa base will not have these factors to such a degree as to limit your ability to shoot the rifle.


-Will this limit my scope options? Some scopes only have 50-55 MOA of total internal adjustment. Are they normally designed with the 0 MOA point being right in the middle of that range (therefore allowing 25-27 MOA of downward adjustment and accommodating the 20 MOA base elevation), or do they typically have greater upward adjustment than downward adjustment?

it will limit your scope options some; you won't want to intentionally seek out the lowest amount of adjustment in a scope you can find. i have yet to put a scope on a rifle and not have to mess w/ elevation knob to some degree to get dialed in, so while in theory a scope is at its finest when the x-hairs are in their true center it just is not practical. most scopes will not show any major issues until you get to within 5-10 clicks from maxed out, so this is not something i'd worry too much about.


-Any other drawbacks I should be aware of?

the biggest one is the height the scope will be mounted. the 20 moa base will carry the eye piece pretty high so you may encounter comfort issues shooting from prone unless you can use some method of raising the cheekpiece a little.

i have a leupold vx-3 w/ a 30mm tube mounted to a 20 moa base on a 308. this scope will dial a dead-on zero at 1000 yards. it will also dial down to 100 yards.

i also have a sightron s-2 w/ a 1-inch tube in 20 moa bases on a 243. i can dial in a 1000 yard zero, and it will also dial in at 100 but it is nearly maxed out at the bottom at 100 yards. however, my standard zero is 250 yards so this is not an issue for me.


...the longest that's convenient is only 200 yards, so I won't get the benefits all that often.
kind of sums up your needs there... not seeing a great reason for you to use a 20 moa base, and if you do it, be aware that some scopes will not zero at 100 or even 200 yards. in some cases you will be stuck w/ a 300 yard zero for the close end, and having to hold 8-12" low (via kentucky 'windage') so you can punch a few targets at 100 yards will not maximize fun.

just pony up for a 0 moa or 10 moa base. i don't think a 20 moa base is in your best interest.

USSR
June 20, 2010, 08:22 AM
I have a 20MOA picatinny rail on 3 rifles. No problems getting a 100 yard zero, and I can dial in enough elevation to go to 1,000 yards on all three.

Don

Walkalong
June 20, 2010, 11:08 AM
How much MOA adjustment does a scope need to do that using the 20 MOA slanted base Don?

Z-Michigan
June 20, 2010, 11:26 AM
Even with a really high-quality Leupold scope, worth about $1,500. - the absolute lowest I could "dial down" was about 30" high at 200 yards. I tried unsuccessfully to sight it in at 200 yards, with a huge piece of paper.

I suspect that the 50 BMG was equipped with a 50, 75 or even 100 MOA base, which is fairly common for 50 BMG and totally out of place on most other rifles.

Good info here, keep it coming if there's anything to add. I may cough up the $40 for the 0 MOA base given my shooting conditions, but I like to know all the options and issues.

Jim Watson
June 20, 2010, 12:14 PM
Let's see, now.
You already have the rifle.
You already have the 20 moa base.
Why not put them together and shoot the gun? It's not going to hurt anything to try.
You will soon see if you have enough scope adjustment to get down to 100 yards.

ArtP
June 20, 2010, 01:55 PM
I can add this:

Think of the internal travel as a circle instead of a square - the amount of travel you have in any direction will be at it's widest point only when the other direction of travel is in the middle. If you are maxed out with elevation, the windage travel will be much reduced because your are at the top (or bottom) of the circle. I hope that makes sense. I'll clarify if it doesn't.

Rings, base mounts and bore alignment are almost never perfect and will require a certain amount of windage to shoot straight with no wind. If for any of those reasons you require a moderate amount of windage adjustment, it may be asking too much to then dial the max amount of elevation, too.

With the rifle I do most target work with, I happen to be lucky and don't need any windage adjustment, which gives me full elevation adjusments. I have had two different scopes mounted and neither required more than one minute of adjustment.

ArtP
June 20, 2010, 02:03 PM
I have also heard of people using perfectly cut pieces of aluminum can under the other base to reduce the affects of the other base, turning a 20MOA base into a 13 MOA base, for instance, or whatever is necessary. I can't remember how much adjustment one sheet of can adds, it could be googled.

Maverick223
June 20, 2010, 02:10 PM
It is pretty well established that you don't really need the additional external elevation...but you already have it, so give it a whirl, and save some cash. Unless you are using optics with little adjustment you won't have an issue, and fear not there are no significant drawbacks to external elevation. I like the additional elevation on most rifles when given the option, so I certainly wouldn't buy a new one without at least trying the one you have. FWIW, I have a 40MOA mount on my LRPR and can get a zero just fine.

I can't remember how much adjustment one sheet of can adds, it could be googled.Here ya go: Scope Shim Guide (http://www.gatewaytoairguns.com/library/Scope%20Shim%20Guide.htm)

:)

USSR
June 20, 2010, 04:04 PM
How much MOA adjustment does a scope need to do that using the 20 MOA slanted base Don?

Typically, greater than 40MOA. And that's assuming your starting in the middle of elevation adjustment at 100 yards, which isn't always the case. You have to be careful with the Bushnell's, as they typically don't have a whole lot of W&E adjustment built into them.

Don

Andrew Wyatt
June 20, 2010, 05:21 PM
theres no drawback to using a 20 MOA base.

Additionally, you should probably consider a 250 yard zero.

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