3/8 mounts the same as 11mm???

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.

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.

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.

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