11mm scope dovetails: Why do they interchange with 3/8″?
by B.B. Pelletier
Before I get started on today’s blog, I want to announce that Pyramyd Air is now making their email campaigns available online.
This is great news, because many of us have avoided subscribing due to already-full inboxes.
Each week, a new email is sent out with new products, sales and promotions, special offers, etc.
They usually come out on Wednesdays, but not always.
Now, on to today’s blog.
dm20 gets the credit for this one.
He made this comment. “I thought you said makers of quality rings only labelled them in either 3/8 or 11mm,
and not both?”
I probably did say that, though I don’t remember the context of the statement, so it’s hard for me to remember exactly what I was referring to. But that’s not the issue. The issue is: What is an 11mm dovetail?
Don’t use common sense!
My aunt once told me that common sense isn’t that common, and she was right. Think about electrical plugs, computer operating systems, cell phone plans or whatever, and you’ll see what I mean. When something absolutely HAS to work, like air transportation, we gag the dreamers, fire the committees and enforce the standards ruthlessly. But, when free enterprise is given room to innovate – watch out! You’ll get digital cameras that don’t interface with most computers, giving rise to entire businesses that exist just to get pictures from your easy-to-use camera onto paper. So it is with the mythical 11mm dovetail!
The B-Square study
I am acquainted with Dan Bechtel, the founder of B-Square. His company grew up making no-gunsmithing scope mounts for military rifles back in the 1960s. In the 1990s, Dan saw an opportunity in airguns. He saw people were mounting scopes on more and more guns, and he wanted a piece of the action. Like many of you, he thought all he had to do was make a scope mount that fit an 11mm dovetail and be done with it.
That’s when he learned the bitter truth. Airgun dovetails – those 11mm dovetails we all talk about – actually range in size from 9.5mm to almost 14mm! When he discovered this, he wasn’t discouraged. He simply made a mount with adjustable clamps that spanned the distance between the high and low number – one size fits all. Except, it didn’t fit all! In fact, it was the reverse. It fit almost nothing very well. The rest of the guns had scope clamps on such an angle that their owners complained bitterly. What was wrong with B-Square? Why couldn’t they make a scope mount that fit the dovetails properly?
I was one of a team of people around the U.S. who measured dovetails for B-Square. We even had to come up with a standard way of doing it so all our measurements would jive, because with a dovetail, where do you measure? From the bottom of the cut? From the top edge? Think about it. It’s not obvious. We standardized by using two short 1mm wire strips inserted in the dovetails and measuring from the top of one wire to the top of the other. They went into the dovetail cuts almost completely, giving us a standard point of reference. If you don’t understand what I’m saying, it doesn’t matter. We measured all the airgun dovetails we could find, so B-Square could make mounts for them.
It gets worse
Well, it turns out there is even more to it than the width of the rails! Some makers cut the dovetails with a 60-degree angle while others cut it with a 45-degree angle. The angle of the cut influences the angle and depth of the clamp going into it, so B-Square had to use clamps with rounded edges as a compromise. Then they took heat because those clamps looked like they didn’t fit ANY dovetails right! But there is even more to it than that!
The profile of the receiver above the dovetail affects how the scope mount fits on the gun. If the rifle is rounded and if it sticks up too high, it can hit the bottom of the mount and make it rock to one side. Nobody likes that. You might think that was the end of it, but there was more.
The scope stop mechanisms on air rifles are not standardized. Weihrauch and Air Arms use vertical holes, Gamo is in a transition from a flat plate, which many Chinese makers use, to a more traditional add-on scope stop. Webley, FWB and CZ use half-round transverse grooves – all serving the same purpose. The scope mount has to be made taking things like that into account.
No hope for some
Then, there are the guns such all the RWS Dianas that, even today, have absolutely NO provisions for a scope stop! We have to be creative in how we mount scopes on these rifles because the factory obviously doesn’t know or care that there is a problem. I think they think you can just clamp to the dovetails real hard and that will solve the problem. I know hundreds of shooters who have discovered otherwise. And, finally there are guns such as the Webley Tomahawk, which has no provisions for scope stops at all! None! Even B-Square can’t do anything about that.
About now is when someone stands up and shouts, “Why can’t they all just standardize on one kind of scope mounting system?” Sort of underscores why the military went out of their way to invent their own system, doesn’t it? So, dm20, 11mm mounts SOMETIMES also fit 3/8″ dovetails just because they do! A smart mount maker tries to fit as many guns as possible for the sake of more sales.