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Education / Training 11mm scope dovetails: Why do they interchange with 3/8″?

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.

No standards!
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!

And worse
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.

And worse!
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.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

31 thoughts on “11mm scope dovetails: Why do they interchange with 3/8″?”

  1. This is why the word snafu will never be obslete. It makes one wonder…are the airgun manufacturers all so arrogant as to imagine everyone else will come over to their way of cutting dovetails? Or do they just figure each of us is only going to buy one airgun so it’s our problem?

  2. I didn’t know the history, but what you say is true. I usually find Beeman mounts fit their guns and usually some others – otherwise it’s unpredictable. I’m usually not happly with economy mounts anyways.

    Springer John

  3. I have noticed that Diana RWS 34 and the 350 models now have improved their equipment somewhat.
    #1. – The scope rail screwed to the reciever is now STEEL vs aluminum. The scope stop pin has been staing put in the detent hole, vs. ploughing a furrow to the rear.

    #2. The RWS C-Mount today is different from the past. It is now a B-Square 17301 mount privete labled for RWS in a box that says RWS C Mount. But it IS identical to B-Squares 17301!

    Love the blog BB.

  4. KTK,

    Thanks for this information! I will now look at all the scope ramps of Diana airguns to see when they change to steel.

    As you may know, RWS doesn’t make airguns, just like Beeman doesn’t make them. They put their name on guns made by other companies, and Diana is their largest supplier.

    So when RWS switches production of their C-mount from another mount maker to B-Square, the model name sometimes stays the same while the product changes.

    In this case, though, it’s even more convoluted. B-Square purchased the Australian company that was making the C-mount for RWS, then they convinced RWS to switch to a different model in the b-Square line. B-Square actually produced the old-style C-mount for several years before the switch was made.


  5. BB, Thanks for your reply.
    With what you say for B-Square, Cudos to them for getting RWS to go with the 17301 as it is a good mount for most scopes (40mm or less obj’s.) I feel that with the 3-9 x 40mm scopes though one should step up to the larger B-Square mount for more clamping power in addition to the stop pin.

    With the B-Square steel stop pin droped in the steel detent of the scope ramp it seems to work. I would feel more comfortable with the detent hole having parrallel taper walls (not with the slight taper inward) though.

    My biggest problem today with my 34 is not scope related but with the Beeman Sport Peep (Made by Williams I suppose?) I can’t get the rear of the peep low enough to shoot at 17 meters or less. Using heavy Beeman 0.22 ram jets it is almost there but still 1/4″ high. I can use the taller front sites off my 350 as it uses a larger and higher front post insert than the stock 34 has, but now my 350 has no front site. O well I use a scope on that gun anyway… Having a lot of fun anyway.
    I got a Tx200 from Pyramyd based your blogs and opinion and I love it. 🙂

    KTK – Racine,WI

  6. boy, you really DO know everything!
    aquainted with b-square, tom gaylord, pyramyd air, spoken with nearly all of the major makers- wow.

    i cant recall exactly what either, but i do know it had to do with gamo labeling their mounts for both. i imagine they get returned alot? its no wonder, then, why b-square mounts are more expensive.
    they’ve done their homework!

  7. B.B.,

    Do you need a Scope Stop for a Beeman R7 with a small scope like the bugbuster? are mounts with stop pins good enough to keep the scope from shifting?


  8. BB,
    Umarex USA has an unbelievable mark up on parts. The 350 front sight is almost as much as just getting another peep from Williams. Williams has 2 models that I am 100% positive that one of them is the Beeman Peep. The FP-AG-TK for high line of sites (this might be the Beeman) or the same unit in low line of sites is FP-GR-TK. If the low line of site one is NOT the Beeman Peep then I can buy it and later use the beeman peep on another gun (350 with No scope?) I need to research this. I could call Williams but normally the Factory that sells and private labels for another Keeps the information on what model they supply to whom very close the vest.

  9. KTK,

    Here’s a thought. Pyramyd AIR wiull soon be selling a Mendoza peep sight that, in my estimation, is as fine as the best Willams peep made. It will also be less money. Let’s wait and see how high it is. I think they will have them in a wekk or tow at the most.


  10. hi bb
    why do airguns and firearms have different rail sizes? the rails probly caried over from firarms so why wouldnt they be the same size.
    also do you think any manufacturers will be putting weaver rails on airgun in the future? the mounts are readilly avalable and it solves the recoil problem

    Field Targetier

  11. I’m just surprised that the dovetail style mounts are still popular. Isn’t a Weaver/Picatinny rail so much more functional due to the integrated scope stops? If I remember correctly, 3.8mm Weaver rails are standard on all NATO rifles and the specifications are clearly laid out so there’s no room for interpretation.

    -Alan D.

  12. I wonder if the companies what their own style? They don’t want to be the same as the competion.

    Sometimes it’s money. I’m sure Sheridan came up with the .20 Cal.
    so you had to buy their pellets back then.

  13. Field Targetier,

    You must have missed my rant a few weeks back on Weaver mounts, Yes they are good, Yes they make sense. And, no, I dont think the airgun makers will adopt them any time soon.

    Airgun mounts are not derived from firearms mounts. They evolved by themselves in the UK and Germany.

    As I said in the post – don’t wait for thinks to make sense, because they never will.


  14. B.B. Off Subject— Why are so many “tuning” their “new” Springers? It seems like I read volumes of tuning spring airguns. Are they not up-to-par when arriving “factory fresh”? Why aren’t they “tuned” at the factories on the production line?

  15. Don,

    Some springers, like the BAM B40 and TX 200, are fine when they come, but many others are in need of a tuneup. Especially the triggers on the Gamos.

    Factories have zero time to fool around with tuning their guns. They make them as best they can and that’s it.

    Some companies do better than others. I try to point that out when I test spring guns. For example, there are a number of very good-looking Argentine rifles you will not see aPramyd Air carrying because they failed in their preliminary test. A couple of Turkish rifles also failed.


  16. wouldnt it be simple to smear some tar onto the spring before sliding it into the receiver? just dip it into a bucket, pull it out and slide it in. for the guns that cost a little more than the cheap springers, i would think thats a reasonable step in making a better gun.

  17. Ah, the joys of the scope. I’ve only owned one gun that was scoped–a Winchester Model 70–and I only had it for a year or two before trading it for a Colt Government Model. Lately, I’ve been considering giving a springer air rifle a try since I’ve never had the experience. I’m also thinking about scoping the springer. After reading a lot of forums and sales promos, I thought the Diana 34 might be the best choice for me since it has a decent trigger and a good barrel and a pretty good reputation–and the price is acceptable. Since I also was considering going with a scope, I have been concerned about the problems voiced about the Diana rifles on rails, mounts and the lack of effective scope stop method. If the new Dianas do come with a better system of scope mounting as mentioned here, my decision is pretty much made. Is it possible to determine when the Diana 34 started being fitted with the steel rails and when the RWS C-mount style was switched? I’d like to be sure the rifle and mounts have the better system.


  18. B.B.–Your info paid off again–How to judge distance shooting up-into trees. I shoot at my sister -in laws house and she asked me to get rid of the woodpecker that was going to town on her siding. Yesterday I was target shooting there (with my portable bench from cabela’s-thanks again for that one)when I saw him hammering the house. Grabbed a couple of loose pellets -gave them a fling and he flew to a near by tree. Put the sites on him -pulled the trigger and-nothing ,clean miss. Then I remembered one of your reports(I’m zeroed at 25yds)-even though he looked a good distance off I looked at the tree and figured it was only 20yds away. Put the sites on him again-shot high to compensate for the distance -20vs25yrds and now the bird is history. It’s amazing what can be accomplished when a little of your knowledge is taken into the field–Thanks Scott P.S.–now word yet on that week long shooting seminar where we fly to a worm sone and shoot from dusk to dawn?

  19. i’ve got a problem- i left my daisy 840 pumped overnight- this is the second time. i’m estimating a velocity of 250 fps, and also estimate the pump effort is about 1/2 of what it was new (i know, that doesnt check in with the laws of physics very well.) the felt wiper is soaked with oil, and in a last ditch effort i put a few drops down the inlet hole. is it possible to buy and replace the seal on this gun myself, or is there a quick-fix for this? its not very accurate or powerful, but i’m starting to realize its value as a plinker. its also some odd $60-$70 CAD. not really much, but enough i’d like to at least try fixing it.

  20. Hello, I am trying to mount a scope on a cheap Chinese pellet riffle. I know what a dove tail mount is, however this one does not look like a true dove tail mount. Is there a special mount I need on for this Chinese riffle? How do you measure the groves? They look a lot thinner. How can I know if I am getting the right mount or not? Please help!!

  21. Frustrated,

    You say you know what a dovetail mount is, but which of the three different dovetails are you thinking about? The Weaver? The 11mm? Or the 3/8"?

    Airguns almost always have an 11mm dovetail, so that will be what your gun has. What about it doesn't look right to you? Are the dovetail cuts too thin? That's also common on less expensive spring guns, but it doesn't hurt the mounting situation. Eleven mm dovetail rings will still grab the thin dovetails.

    Many airgun mounts these days are made to also work on 3/8" dovetails, which are common on .22 rimfires. So don't be sur[prized if the mounts say they fit both. They probably do, but the base may sit cockeyed on the gun. That doesn't matter, so long as the scope tube holes align, which they almost always do.

    Please get back to us with any questions you have.


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