Rifle Scopes Rifle Scope Ring Height and Clearance Calculator

[GunDudeUSA]

Fellow hides,

I noticed that scope ring height calculation is a recurring request here in Sniper's Hide. Problem comes when the availability of information is scarce and you sorta have to run around the Internet searching for the answers. Anyway, I thought I'd consolidate some of the info I found and made a simple calculator for all future requests.

>> http://www.mil-rad.com/scope_ring_calculator <<

Let me know what you think. It's got some of the pre-saved scope and ring info saved already i.e. Seekins, Badger, Spuhr, Schmidt & Bender, US Optics. Select a scope and it finds the objective bell diameter for you. Select a ring and it finds the ring height for you. If it doesn't have what you're looking for, you can just manually put in what you think the measurements are. Also accounts for Butler creek cover.

I'm still a newbie at this (long range shooting that is). So feel free to offer suggestions, corrections, recommendations on what else you'd like to see in the tool. Thanks!

 

[courtier]

That's an awesome tool and really helpful for those of us -- like me -- who are still trying to figure out sizing. Kudos!

 

[P3T3R]

This should be a sticky thread

 

[Gasfish26]

Awesome calculator
Thanks

 

[Rob01]

This should be a sticky thread

There is a sticky on this subject. The first one at top of page.

 

[acudaowner]

just to ask do they make this for other scopes , thank you for sharing the page i was stuck using stacks of pennies before maybe next time I may be able to use this .

 

[Rob01]

Nothing to do with a particular scope but the dimensions. The more you do it the less you need pages like that but they are good to give people an idea of what they need.

 

[fastfwd]

Awesome calculator

It's good for certain setups but misleading for others. For example, I can practically guarantee that you'll get the wrong answer if you have a canted rail or mount.

Kudos to @GunDudeUSA for stepping up and making that calculator available, but I wish he'd followed up and corrected its flaws, especially since it's so frequently recommended to people asking about scope clearance.

 

[gnochi]

It's good for certain setups but misleading for others. For example, I can practically guarantee that you'll get the wrong answer if you have a canted rail or mount.

Kudos to @GunDudeUSA for stepping up and making that calculator available, but I wish he'd followed up and corrected its flaws, especially since it's so frequently recommended to people asking about scope clearance.

I mean, that is specifically addressed on that page, and also has an offset calculator...

 

[fastfwd]

I mean, that is specifically addressed on that page, and also has an offset calculator...

Yes. You'll almost certainly get the wrong answer anyway.

 

[Rob01]

No you won't. The amount of difference is a 20 moa or even 30 moa is pretty small and will hardly enter into the calculation.

 

[fastfwd]

No you won't. The amount of difference is a 20 moa or even 30 moa is pretty small and will hardly enter into the calculation.

For a Vortex Razor HD2 4.5-27x56 with sunshade, a 20 MOA Spuhr mount reduces clearance by just over 2mm and a 30mm mount reduces clearance by just over 3mm.

I don't think it's accurate to characterize that as "pretty small", especially when the calculator gives its results in hundredths of a millimeter and offers a whole secondary page dedicated to eliminating even "0.89 mm" of tangent error. People who use the calculator want to mount their scopes as low as possible -- if they didn't care about that, they'd just buy the tallest available rings and be done -- so even a 2- or 3-mm error can mean they spend $400 on a mount that doesn't fit. And it's easy to get results with even larger errors.

But the magnitude of the error isn't even the point. My issue with the calculator is that the errors could be eliminated just by writing better instructions. The internal arithmetic is fine; it's the input to the calculator -- the numbers entered by users who are following the instructions on the page -- that lead to the inaccuracy.

 

[Rob01]

2mm is nothing. People who want the scope almost touching are usually people who are told that is the way it has to be by the internet and need to learn it's not the case.

And it's not about buying the highest but getting the clearance. So you can be over anal about 2mm but in the real world it doesn't matter.

 

[fastfwd]

you can be over anal about 2mm but in the real world it doesn't matter.

I agree that in the real world, mounting the scope with a few millimeters more clearance is ok. It might even be better, if your stock makes it hard to get your eye real low or if your scope has a deep erector housing and you're using a one-piece mount.

Similarly, in the real world a couple dollars won't make or break me. But if I go to the store and buy a $2 item, hand over a five and get $1 back, I probably won't just shrug and say, "Yeah, close enough."

If the calculator's errors were due to numerical rounding or component tolerances, or because the published dimensions it relies on were wrong, that would be one thing. But that's not the problem. The issue -- just as with calculating the change due on a $5 payment for a $2 item -- is that it's easy to get the right answer, so it makes no sense to be given the wrong answer for no good reason.

Here are two examples of what I'm talking about. There are plenty of other similar issues:

1. Say you have a 20MOA base -- just a scope base over the action, not a full-length rail -- and a tapered barrel. You're using an S&B scope and Seekins rings, both of whose dimensions are correctly built-in to the calculator. So reading the instructions, you see that all you need to do is measure the barrel-to-rail height (at the point on the barrel where the objective bell will reach, since it's a tapered barrel) and do the calculation on the main page, then go to the secondary page to add in the tangent loss from the 20MOA base. Right?​

​

Well, no. Because how are you going to measure the barrel-to-rail height 7-9 inches forward of the front of your base? Obviously, you'll lay a straightedge on the base, extend it forward to where the objective bell would be, and measure there. And then you'll go to the secondary page and calculate tangent loss, and no matter what number you get there, it will be wrong, because by measuring with a straightedge on the base you've already included the tangent loss.​

​

If the instructions said to always measure barrel-to-rail height with a straightedge extended to the objective bell, and that if you have an angled base you don't need to further compensate for its tangent loss, you'd have gotten the right answer.​

​

2. Say you've got a 0MOA full-length rail, and again all your components are built-in to the calculator: US Optics ER-25 scope with Butler Creek caps and a Spuhr SP-4802 mount. This one's super-simple: Just pick the components from the menu, set barrel-to-rail height to 0 as the instructions instruct, and hit the Calculate button.​

​

The menu of Spuhr mounts shows the 0MOA, 20MOA, and 44MOA versions of each height, and each part number's height and angle are explicitly enumerated. So obviously, the calculator knows the difference between the mounts and will take your mount's angle into account. And therefore there's no need to use the secondary page to calculate tangent loss. Right?​

​

Wrong. The calculator surprisingly gives the same result whether you choose the 0MOA, 20MOA, or 44MOA version of a mount. In this case, it'll tell you that your scope will fit with 1.35 mm to spare, but unfortunately the scope will actually be a couple millimeters too low. Which sucks, because the SP-4802 is the tallest Spuhr mount on the list, so even if you wanted to choose the next size up "just to be sure", you don't have that option.​

​

A simple change to the instructions, to let users know that the main calculator doesn't compensate for the mount's angle, would prevent this error.​

Like I said before, kudos to the guy for putting the calculator online. But its design makes it easy to get the wrong answers, and it's too bad that it was never updated with the small changes that might have fixed that.

 

[Tweedycarcass23jonb]

I know I can’t get the damn thing to work right ?, but I appreciate having the tool. I just went with highs.

 

[TheOE800]

Looks like the website is down now. Been using it for years.

 

[Custom700]

Still there.

https://mil-rad.com/scope_ring_calculator

 

[TheOE800]

Still there.

https://mil-rad.com/scope_ring_calculator