Calculating Minimum Scope Ring Clearance


July 18, 2009, 09:34 PM
Is there a webpage calculator that can tell me the minimum height that my scope rings need to be for a given tube size and main objective size, and base cant?

Barring that, can someone check my math? I have a 30mm tube with a 50mm main objective. That means that 50-30=20mm sticks out of the scope rings. Dividing that in half gives 10mm between the bottom of the scope ring hole and the barrel. So my rings need to be 10mm high for clearance, right? That number doesn't include the thickness of the base, which will help.

Just to add a little confusion if I did the previous calculations correct, I'm looking for a 20 MOA canted base, so it no longer runs parallel to the barrel. a 20 MOA is 1/3 of a degree. The tangent of 1/3 a degree is small, but not negligible. Multiplied by 7 inches (the estimated hang-over length of the scope), and then converted to millimeters gives just over 1mm that I need to account for.

So I need a total of 11mm from the bottom of my ring to the bottom of the hole in the rings?

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July 18, 2009, 10:22 PM

It's not quite that easy. Different barrel contours come into play as well. Tell me what scope and rifle you are talking about.


July 18, 2009, 10:54 PM
Yea, I see what you mean. I am talking about a Remington 700 sps varmint .308. The barrel seems to taper down a bit adding a little bit of clearance, but I was going to save that as a safety margin.

I am looking at putting a Bushnell Elite 6500 with a 50mm Objective on it. The tube is 30mm.

July 18, 2009, 11:41 PM
I know it takes more time. But the best way is to get the scope and then install it on clay rings. Check the clay and buy the rings. The way barrels contour your eye relief may be just enough different then someone else's to have to move to the next taller or shorter ring height.


July 19, 2009, 08:29 AM
More than likely your going to need High rings for that set-up

July 19, 2009, 08:43 AM
I agree that high rings are probably needed. Most companies that manufacture mounts have a webpage listing the type of mounts and what size scopes they work with.

July 19, 2009, 10:57 AM
Well I can't do math anymore, but 40mm objectives generally get medium height rings, 44-50 mm objectives get high, AR's get high or extra high. Seriously, all the manufacturers make them slightly different heights. Since I try to use Weaver or picatinny bases on everything now I just keep at least one set of each height on the bench and try them until one fits right.

It sounds like you are on the right track though, it's just too complicated for an old fart.

July 19, 2009, 11:12 AM
I was looking for a number for just those reasons, each ring maker's medium/high is different. I think I can get away with a medium ring, but before I drop alot of money (for a set of rings), I want to make sure they will fit. Meanwhile, I don't yet have the scope to try this out.

July 19, 2009, 11:43 AM
This is why I have baggies of different sets of rings at very different heights.


July 19, 2009, 04:23 PM
If you are going to use a 20MOA rail, standard rings will most likely work. But you can contact these folks and ask them.

July 19, 2009, 05:20 PM
I think I can get away with a medium ring...

You are correct. Some companies call them standard. In any case, you don't need high rings.


July 19, 2009, 06:47 PM
Measure the outside bell of your scope to be sure. I'm running a 20 MOA on one of mine with a Leupold. I've got about 2-3mm clearance.

July 19, 2009, 07:50 PM
my 30 mm tubed, 50mm objective scope on a 20moa base on a rem 700 vls sits in low rings...

July 19, 2009, 07:52 PM
Thanks for the help. What rings and bases are you guys using? I was looking at Warne one-piece 20 MOA base and Burris Xtreme Medium Rings. But I'm open to all suggestions. I know this is the critical point for the rifle so I'm in the 100-200 range for some quality stuff.


July 19, 2009, 08:36 PM
I've got Badger Ordnance 20MOA picatinny rails on all my rifles. Various tactical rings (Badger, Leupold, and TPS).


July 20, 2009, 05:29 AM
Iron Brigade Armory one pc scope mount, and Farrell rails and rings.

July 20, 2009, 04:28 PM
Take your rifle and your scope to the gun shop and pick the rings that allow you to mount the scope as low as possible 1) without touching anything on the rifle and 2) clearing all moving parts.

I have never seem a ring height calculator, seems like that's not something that would be useful but if you go to MidwayUSA's site they have a ring height selection chart that maybe useful to you.

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