What scope ring height to clear BUIS?

[Artery]

Anyone know the minimum scope ring height to clear a ARMS 40L BUIS?

Also, would would the suggested level be when using a standard M4 stock.

Scope in question is a Leupold 3-9 VX-1

Hope this hasn't been asked before.

[aplinker]

I'm betting high or extra-high will be what you want.

A standard cheek weld, depending on the ring company, is with high or extra high.

[maxicon]

Clearance depends on the ring height, mount height, and eyepiece diameter. Also, many people want the optics centerline to be somewhere around the iron sight centerline (1.4" above the rail), so you can use the same cheek weld and shoulder mount with both for faster shooting.

Another issue is forward offset. Many people need the scope to be further forward than just rings on the rail will give them in order to get good eye relief, and have to use an extended rail or a one-piece mount. This depends on your stock and shooting position, and the best reference is to see where you need your eyepiece to be relative to the charging handle.

Here's how to figure out what height will fit.

- Measure the scope's eyepiece diameter, with flip caps if you're going to use them. Butler Creek usually adds about 0.1" to the diameter.
- Divide that by 2.
- Add that to the height of the BUIS.
This gives the minimum centerline height rings that you need for your BUIS to clear your scope.

According to ARMS, the ARMS 40L height is 0.61" folded. The Leupold VX1 3-9x40 has an eyepiece diameter of 1.6", according to Leupold (they have a great website for this kind of data).

So, (1.6/2) + 0.61 = 1.41" - this is the centerline height of the rings or one-piece mount you'll need. With flip caps, it'll be more like 1.46". Most rings in this height range will be called ultra-high or super-high. Note that many vendors spec ring height to the centerline (like Leupold), while others spec to the base of the ring (like Burris), and you need to add 0.5" (1" tube) or 0.59" (30mm tube) to get the centerline height.

Here's the Big List of AR15 one-piece mounts, including sections on using rings on an extended riser or only rings on the receiver:

http://www.maxicon.com/guns/optics/one_piece/ar15_onepiece_mounts.htm

As an example, the setup below has the Leupold clone scope way too far back for me to shoot with - I need the eyepiece a bit forward of the charging handle. This position works for some people with the M4 stock at full extension, but not for me with the fixed stock.



This position works much better for me - note the rings on the extended rail:

[Artery]

This is brilliant, thank you so much