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Mount Rings for Rifle Scopes

What mount rings should you choose when buying for a rifle scope?

This guide tells you how to get it right.

Key factors – In brief. More detail further down.

  • Rifle base type/Width > Mounts base fitting type
  • Scope tube diameter > Mount ring diameter
  • Scope objective lens size > Mount height profile

Secondary factors

  • Rifle recoil > Can influence mount design, fixing type etc.
  • Rifle stock/barrel configuration > Can influence mount height profile choice.
  • Is rifle a known barrel drooper? > Can influence mount model and/or require extra base accessory.

Key Factors expanded upon

Rifle base type/Width > Mounts base fitting type

Any rifle that can have a scope fitted has some sort of base rail on top of the body where mount rings can be fitted.

In most cases it is one of 2 types – described below. 

Note that there are other types of base that are manufacturer specific – Things like 15mm width or “drilled and tapped” . The cynic is us believes this is to ensure nice extra profits for the manufacturer by locking the user into their own often very expensive mounts! If your rifle does not match either the airgun or weaver/picatinny profiles it is best to check before buying. Ask around in user forums or go to the supplier of the gun to ask.

Airgun 3/8″ dovetail – generally 9-11.5mm width between twin dovetail grooves.

  • As name suggests found mainly on airguns – also common on rimfire, .22s and many pistols.
  • Some airguns (notably many BSA models) have 13mm width. If yours measure 13mm be careful when buying as many airgun mount fitting plates will not go out to 13mm. Be sure to check the model you are looking at goes to 13mm. If it just says 3/8″ then it probably does NOT. If unsure ask the seller.

Airgun base

Weaver/Picatinny 20-22mm

Originally and still is a military derived base type. By design very strong and proof against any sort of slipping from repeated recoils.

  • Found on military designs and many centerfire and hunting models. Also appearing on more airguns especially high power recoil models.
  • Approx. 20-22mm width base with horizontal grooves across.
  • Mounts feature bars that slot into the base grooves ensuring a rock solid fit and zero recoil shift.
  • Essential to ensure what if you have a 20mm width base that it has the grooves. Some rifles are 20mm across but no grooves. This indicates a specialist mount is required.

Weaver – Picatinny base

Scope tube diameter > Mount ring diameter

Easier than the bases but often a reason for returns due to not reading of the advert or knowing that it is even a factor.

  • Most scopes have either a 1″ (25,4mm) OR 30mm tube diameter.
  • Simply make sure that you know what your scope is and choose mounts with the correct ring diameter.

Scope objective lens size > Mount height profile

  • Mounts are described also by height – meaning how high they raise the scope from the body of the gun.
  • Generally you will see extra low, low, medium, high and extra high.

Height is important for 2 reasons

1) To raise the scope high enough so that the large end lens (objective) does not touch the gun body – clearly not a good thing! Most users want it a few mm above the body.

2) That it is not so high above that the scope elevation control runs out of movement before the scope can be correctly zeroed. You do not for example, use an extra high mount that will suit a 60mm lens scope with a 24mm lens scope. It will sit the scope so high that you probably won’t be able to adjust it correctly.

As a general rule – but can vary (see secondary factors)

  • Low and extra low suit 20-30mm lens scopes
  • Medium 30-44mm
  • High 42-56mm
  • Extra high 56mm+

Most scopes used in the general hunting and targets market will use a medium or high mount. Most of the time if you need a different type you will already know it or the mount will have been provided with the scope.

How height profile is measured and how it related to scope lens objective sizes

In addition to being described as medium, high etc and good advert will also quote the height of the “saddle”

The saddle is the distance from the base of the mount bottom to the bottom of the ring. This is useful to know if you need a really specific height.

For example

  • 15mm saddle (a common “Medium profile” height with 30mm ring
  • Means that from the bottom of the mounts to the centre of the ring (radius) we have 30mm of total height from the base of the gun.
    • 15mm saddle plus 30mm diameter/2 = 15mm radius.
  • Therefore the largest diameter scope that can fit here without grounding out will be just under 60mm.
  • But this does not mean a scope with lens 60mm – oh no.
  • It means the scope outer casing diameter of course. A 50mm lens scope for example is surrounded by it’s protective casing – and maybe there are flip up lens caps in use.
  • This can add anywhere from 5-15mm to a scope lens.
  • So a 30mm ring scope with 15mm saddle will most likely be very tight on a 50mm lens scope.
  • So measure the outer lens casing before deciding on height.

Secondary factors

Rifle recoil > Can influence mount design, fixing type etc.

If the rifle is a big kicker then you want really strong mounts. At least twin screws on the base fitting and top plates.

The latest QD (Quick Detach) models are also very good for big recoils as they lock down very hard – but can also be taken off really easily negating the need to screw them on so tight that they are hard to get off.
Rifle stock/barrel configuration > Can influence mount height profile choice.

If the rifle is such that the end of the scope will hand out over the barrel and not be over the gun body. Or if it is raised on some sort of extra base or rise – Then you have greater flexibility with mount height.

For example –

  • A 15mm saddle 30mm ring mount grounds out a scope with 62mm casing diameter on a gun where the scope sits over the main body.
  • On a gun where it hangs in space over the barrel it is perfectly OK.
  • be sure to consider this as it will allow a tighter fit to body and give more elevation flexibility.

Is rifle a known barrel drooper? > Can influence mount model and/or require extra base accessory.

A few air rifle makes – notable (or notoriously) Diana have barrel that points downwards ever so slightly. To the eye it is hardly visible – but when using a scope at range and trying to zero impact point of barrel and scope crosshair it can make things very difficult.

If you have a barrel drooper either get a barrel droop compensating mount base (available for Diana, made by Leapers UTG) or get an adjustable mount where one or both ends can be raised to counter such effects.

This is not the be all and end all of mounts – we won’t go into things like reach forward and back mounts or offsets or risers etc but I hope it gives you a broad view and helps you choose a mount – hopefully with a scope from us here at tactical scopes

 

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3 Comments

  1. Peter Andrew

    I was fascinated to read your bit about barrel droop compensation, and amazed to find something which describes my problem so exactly, This is exactly what I need, my old Relum which I love has a marked, even visible barrel droop, and I run very rapidly out of elevation on the scope, so that I have to aim very high at over 25m.

    Can you sell me something which would I presume would go between the breech and the scope to compensate for the barrel droop ?

    Peter

  2. bookmarked!!, I really like your blog!

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