Leapers base for RWS Diana rifles - Part 2
by B.B. Pelletier
In the beginning, we learned a lot
When I started the Leapers base for RWS Diana rifles development project, I didn't know what all the variables would be, but that is to be expected in the beginning of any development project. Often, something that sounds dirt-simple will surprise you with nuances and variables you couldn't imagine before doing a little testing. Which is why the first two batches of prototypes I tried were complete failures. But, we did learn a lot of useful things from them; the most important was that an obvious approach toward making this base will absolutely not work. I'm not going to explain what we discovered, because Leapers deserves some consideration for all the effort they put into the project. It's enough to say that the obvious approach to solving this problem of barrel droop on Diana rifles does not work. Learning that lesson added several months to what I had expected to be a quick development.
What range would work?
Once we did discover a method of droop correction that worked, we uncovered several more surprises. Leapers allowed me to control the technical specs we were after, which shaved a ton of time off the development. I didn't have to test anything to know the ranges we were after with this base. I knew we wanted an initial zero of 20 yards, which will put all of these rifles back on zero at 30-36 yards. There is no other distance at which the trajectory is so flat within the useful range of any of the Diana spring rifles.
No two scopes alike
Now what I mean by "an initial zero of 20 yards" is that I wanted a scope base that put the scope at zero at 20 yards when the vertical reticle was adjusted somewhere near the center of its range. Every scope on the market has a different amount of adjustability range (the number of inches of adjustment from one end to the other), but they all have a center. If I could determine the best slope for every scope, so that no matter what scope was mounted they would all be approximately in the center of the vertical adjustment range, then airgunners would have the maximum number of clicks in both directions once they'd obtained a 20-yard zero. And, since the 20-yard zero really puts them on target from 20 to 30 (or even as much as 36 yards), depending on the velocity of the rifle and the pellet style used, this setup would give them the absolute greatest flexibility with their scope, regardless of the make or model.
Not everyone is an airgunner
This is not an insignificant fact. I have heard through the grapevine that some European designers have worked on a similar problem and selected 10 meters as their initial zero point. Ten meters is the worst possible zero point, because the scope and the barrel are not in alignment and must be made to converge. The scope has to be slanted downward to intersect the pellet in flight. By choosing a 10-meter zero, a scope would have to be on such a severe slant that much of its vertical adjustment would be wasted. You'd get only one or two yards with the pellet at the same height. With a 20-yard zero, the greatest amount of useful adjustability remains and the trajectory is the flattest over the longest distance.
What does useful range mean?
To determine useful adjustability, I took into account the fact that very few airgunners will ever attempt to shoot their RWS Diana rifles at ranges beyond 100 yards. In fact, very few will ever intentionally shoot past 50 yards. The fact that the gun will shoot farther is mitigated by the fact that nobody will use it that way. I selected the range within those limits over which the airgunner experiences the flattest trajectory, and they can adjust the reticle if they want to shoot at other distances. With that slope built into the base, the scope always has the adjustability for the other ranges.
A lot of drop!
When I discovered the amount of drop we had to build into it at 20 yards, it was stunning! The barrel was pointing so low we had to make a slope of more than 20 inches to get the scope to the center of the adjustment range with the reticle zeroed. I tested and retested to make certain the slope was correct, but it was. Then it hit me: What if all RWS Diana rifles don't have the same slope as the RWS Diana 34 Panther I was using for testing?
In fact, no RWS Diana air rifle has exactly the same barrel droop as any other RWS Diana rifle. They're each unique unto themselves. Diana doesn't intentionally build barrel droop into each gun. It just happens because of how the rifles are designed...mostly how the breech is made. I won't go into the reason why they're like that because we don't have time for it now, but a study of the design will reveal a barrel slope that's consistent within certain limits. Unless the barrel has been intentionally bent, it will always droop within those limits. However, that's true only for the breakbarrels.
The fixed-barrel models also droop but not for the same reasons and not by the same amount as the breakbarrels, so I also tested the new base with an RWS Diana 460 Magnum rifle - an underlever rifle whose barrel is pressed into the receiver rather than into a pivoting baseblock. Because of this, the amount of downward slant may be more calculated than with the breakbarrel models. If the fixturing were totally random, I'd expect to find rifles that shoot high as well as low. While there are a few Diana fixed-barrel rifles that do shoot high, they're rare compared to the number that shoot low. In fact, you can just about bet that a Diana fixed-barrel will shoot low, though the amount of droop is less than that of the breakbarrels.
Not everyone needs one
Many scopes can actually be adjusted for the droop that a fixed-barrel RWS Diana rifle has, and because of that there are a lot of airgunners who don't even acknowledge the problem. Their scopes are all adjusted up nearly as high as they'll go, but they're on target and therefore satisfied. That may be all they need. If they never shoot beyond 40-50 yards, they may never run into the upper limit of their scope's adjustment. Those shooters don't really need this new base. But breakbarrel owners will hit the limit much sooner than fixed-barrel owners. Plus, everyone who wants to have some scope adjustment remaining will want the new base.
So what about the fixed-barrel owners? Do we give them the same base as the breakbarrel owners, or do we make a base just for them? The decision was up to Leapers, and they decided to do it right the first time. They made a second base just for fixed-barrel guns that was engineered to the slope I measured on the 460. I checked myself and tested the base on the 460 magnum as well as a sidelever RWS Diana 48 - another type of fixed-barrel Diana rifle. Like I said earlier, no two rifles will ever be exactly the same except by coincidence, but I did establish that the breakbarrels tended to be close to each other and so did the fixed-barrels.
All this testing was proven in the third prototype which performed well, except that Leapers wanted to establish that the recoil shock shoulder was a better design than a vertical adjustment screw, so they built two different prototypes - one with the shock shoulder only and the other with the vertical pins only. Of course, by this time, there had to be a prototype set for breakbarrels and a second set for fixed barrels. I had to test both types of recoil restraint in the third prototype, and that added time to the development. In the end, the shock shoulder proved to be the superior design, and the vertical pins weren't needed.
I took the third prototype to the 2008 SHOT Show and talked with the engineer who was doing the design development at Leapers. We could have gone into production at that point, but Leapers wisely wanted to test the base as thoroughly as possible, so they built a fourth prototype that was essentially the finished base except for some cosmetics.
Fourth time's the charm!
On the fourth prototypes and also on the production bases, Leapers put both types of recoil stop - the shock shoulder and the vertical pin. Shooters who feel more comfortable with a vertical pin can have one, though everyone has to use the shock shoulder. It's really all you need. The base absolutely cannot move once it's correctly installed, because the recoil shock shoulder bumps against the front of the rifle's base and comes to a dead stop. It's the same thing we did by hanging the vertical stop pin in front of the rifle's scope base all those years. Now, there's a huge bearing surface, and the new base very nicely conceals the fact that the shock shoulder is hanging over the rifle's base.
I think in my next installment I will share some of the astounding test data with you (targets used to determine drop), plus I'll give you a review of installation - what little there is to say. This new base is the fastest, most reliable scope mounting system ever developed for a spring air rifle, and it will cut your scope installation time in half or less. Far less if you were also using adjustable mounts to get rid of the droop.