My understanding is that 3/8 and 11mm are not the same, not only is the diameter different (although I read that if measured at the same point they are...?) but the angle is also different, 11m is 60 degrees vs 45 degrees for the 3/8.

With that in mind I emailed a supplier outdoorsupplies.co.nz to ask if they also sold the scope with an 11mm mount.  This was there reply "The 11mm and 3/8" dovetails are the same thing, they dont make two sizes its just some call in imperial and some metric"

So now I am a little confused. I don't think the supplier is correct but I have read other people saying the same thing on various forums.  Can anyone shed any light for this for me? If I have 11mm dovetails will I need to buy additional 11mm mounts and discard the 3/8 mounts that come with the scope?  I'm looking at dampa mounts but they are quite pricey and I don't know if I will need them, the rifle is shooting at around 13.5ft/lbs and has the gamo recoil reducing rail thing by the looks of it.

Thanks

"But the difference between these two standards is not only in the measurement unit with which they are named. The “11 mm” in the European dovetail marks the width of the dovetail measured across the top, while the 3/8” in the American dovetail marks the width of the dovetail measured across the bottom as shown in the picture below. The American dovetail actually measures 12.7 mm across the top which means that it is wider than the 11 mm dovetail.

Gamo rail leave on or take off no difference but can be useful for sight height/comfort.

The rail will move the stop hole forward 25mm which you may need or not.

The Nikko Stirling scope is a good length so you should have no issues from either location.

One piece mount with stop pin - good choice

When I look at the Gamo and NS mounts the angles and possibly some measurements are slightly different but both work on 11mm rails in this case so I believe you have been advised correctly.

Nikko Stirling Scope - purchased after 2018 so Lifetime original owner warranty will apply so keep all documentation, removes all risks really.

Thanks mercs for the quick and very helpful reply. I have been reading your thread "Sub $400 scopes with adjustable objectives" as a starting point on which scope to buy.  You don't mention the Air King 3-9x42 but describe the 4-12x42 as "...indestructible junk really and would not be without one." The 3-9x42 Air King fits my budget and what I need from a scope, and is also one of only two air rated scopes for sale at the dealer I'm picking my rifle up from, so is an obvious choice.  I'm glad to hear that I ought to be able to use the mounts it comes with.

If I have any issues wjen mounting the scope I know who to ask. I've mounted several scopes before but mostly on pcps and with no stop pin or aditional riser/recoild reduction device.

I have been impressed with Tasco and more recently ZeroTech scopes for surviving on my springers but tend to fit them with SportsMatch Dampa mounts for a bit of protection so an additional $160 per setup and they are currently unavailable until March. 

I want the lifetime warranty and know that Nikko Stirling, Tasco, ZeroTech and Bushnell will meet their commitment. 

That could be a good post - worst scope disaster and warranty return tall tales.

I could not find a picture of the underside for a later model AirKing mount. 

Using the pointed grub screws on the aluminium rail is probably fine and I expect they may line up with a predrilled hole and could possibly act as a stop pin though on mine the threads are not long enough.

When mounted directly on the  rifle chamber there is a real risk they will damage the finish and highly likely to move and gouge the metal and I recommend removing the screws completely.

For anyone interested - Where there is no stop pin or stop hole then clean the mount and dovetail grooves then Loctite sparingly along the contact face, I have done this with a 4 screw mount on a high power Crosman and it never moved and it also disassembled easily. Not a cheap option buyng cleaner and loctite but less risk than drilling or machining rifle parts.

I had a set of rings that were 3/8" and 11mm.

The side clamp plate was marked with a raised 'dot' that showed the 3/8" fitting.

Reverse it around to fit 11mm dovetail.

But...the actual shape (profile) of 3/8" and 11mm is different too.

The 3/8" bits might (sort of) fit the 11mm, but will not be a nice matching shape that would allow a good tight connection.

The side plates that do the clamping may be made with the correct shape depending on which way you fit them, but the other side of the ring or mount has a sort of 'hybrid' shape that fits either shape dovetail groove. Not a good thing to use.

For my use (benchrest) I only use actual 11mm fittings. I want my scope firmly and accurately held.

For a fun gun and plinking it probably makes little difference...although if you have an expensive scope you would really want the scope/rifle connection to be a good solid fit.