Aligning a scope with the axis of the rifle bore
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Texas Airgun Show
- The question
- The bad news
- The barrel
- Is the scope base parallel to the axis of the bore?
- What about side-to-side?
- Scope mounts
- The answer
- Greater precision?
- Close enough
- What is meant by tweaking?
- Never perfect
Texas Airgun Show
The 2019 Texas Airgun Show will be held on Saturday, June 22. Here is the website with information. This year they will offer FREE TABLES for people bringing a few airguns to the show! In the tent outside the entrance to the hall there will be several tables that are available to people who bring several of their guns but don’t have tables. These are the guys who normally walk the show holding onto airguns they want to sell and trade. You can now put those guns on these tables for free. There will be lots of table sharing going on, and you need to bring everything you need, because these tables are bare. This has never been done at an airgun show before and the promoters are hoping it will help those carrying their guns around to lighten the load.
Now on to today’s report.
I have a Godfather of Airguns webpage, and sometimes people who read this blog ask me questions there because I guess they can’t figure out how to post them here. Today’s question comes up a lot and is worth a discussion. Here is the question.
“I have a question about the axial symmetry of scope mounts.
“Recently I bought an HW50S and I was looking for a scope mount. I decided to buy a BKL 263 two-piece mount because someone told me that this mount is perfectly centered to the axis of the airgun rail, and will not give me a horizontal error at different differences. Unfortunately the BKL was moving on my rail because of the lack of a stop pin.
“Then I decided to buy a good one-piece mount with a stop pin. But before that I checked my other Chinese one-piece mount. I mounted the BKL and that mount on the same rail and then I put a small BB at the bottom of each mount ring. I noticed that the BB on Chinese mount was displaced, relative to the BKL’s BB. Then I turned around the Chinese mount and did the same test. The BB was then displaced to the opposite side, so it means that the mount is not aligned with the axis of the rail. Actually it was near 2 mm of displacement. If the mount is for 9-11 mm rail it’s probably good for only a 9 mm rail.
“So here is my question. Will the Sportsmatch mount be axial to the rail, because I’m afraid it won’t? What should I do? Is that really important to have axial mounts on your airgun? Should I use a file and extend the not moving jaw of the mount to make it more axial?
“I was searching for your posts about that issue but I didn’t find any. If there is one, please send me how is that called or link to that post.
“I will be grateful for any reply. Thanks in advance for your time.
What a question! Matt knows what many airgunners have discovered — scopes don’t look at exactly the same place that the bores of their rifles do. As you shoot close or far away, the pellet will move from left to right or vice-versa. I have written about this several times in the past but today I’m addressing it again. Matt, I never used the term axial, so that may be why you didn’t get any hits.
The bad news
Matt, the bad news is that no rifle on this planet does what you ask. I will get into the reasons for this in a moment, but don’t take it too hard. You asked if this is that important and the answer is no, it’s not. You can work around it and get what you want, despite it being virtually impossible to align a scope optical axis with the bore of a rifle.
Here are most of the many reasons why scopes and rifle bores are never precisely aligned.
We will start with the barrel. It is virtually impossible to drill a hole and rifle a barrel so that the bore is parallel to the outside of the barrel. But no matter because it’s not important. However, there are some anal benchrest shooters who think that it is important and they have their barrels machined outside to be parallel with the bore. This costs a lot of money (hundreds of dollars) and does absolutely nothing for accuracy. Know why? Because the place where the barrel joins the action of the gun determines where the bore “looks,” not the outside of the barrel. The center of the bore can be offset a quarter-inch from the center of the barrel and make zero difference in where the gun shoots.
Now, the place where the barrel is joined to the action does matter. That is what determines where the bore “looks,” relative to the action. So, time invested in getting that right is time well spent. But there is a fly in the ointment of today’s question. What kind of air rifle is Matt trying to scope? A Weihrauch 50 — HW 50. That’s a breakbarrel rifle, and we know that every time that rifle is cocked the barrel (and therefore the bore) moves, relative to the action of the gun.
When a breakbarrel rifle is cocked, the axis of the bore and scope diverge.
However, this movement doesn’t matter as long as the barrel returns to the same place every time. And it does. So we can ignore the fact that the bore moves. Let’s move on to the gun.
Is the scope base parallel to the axis of the bore?
The answer to this question is no — most of the time the scope base (11mm dovetail grooves cut into the spring tube in the case of the HW50) is not parallel to the axis of the bore. I can illustrate this with a term we all know — barrel droop. Those who shoot breakbarrel air rifles know that most of their barrels point downward, away from the axis of their scopes. It’s the reason that droop-compensating scope mounts are so popular today.
This rifle has a droop that’s extremely large, but it illustrates the point I’m making.
What about side-to-side?
I have only talked about the relationship of the scope and bore up and down. What about side-to-side? That can be off, as well, though it’s not commonly as big a problem. But some airguns have scope bases that are attached to the top of the spring tube, and those bases can be attached incorrectly, so that side-to-side becomes a problem. The fix for this is not the same as for up and down, because the trajectory of the pellet doesn’t come into play. In short, on a side-to-side problem gravity isn’t an issue. But it still needs to be corrected.
Now that we understand the problems the gun presents, we still must consider the scope mounts. If the holes through the rings aren’t aligned with the ring bases everything else can be good and we still won’t get the scope and barrel to look in the same direction. The solution here is to use quality rings. And two-piece rings give you options for alignment refinement because you can turn them around or swap them on the rifle (front and rear). You can even swap them and turn them around individually, which gives an even greater range of adjustability. Matt mentioned doing this, so he understands the finer details of scope mounting. But now I want to stop talking about hypotheticals and get down to the answer.
Matt — let’s go back to those BKL 263 scope rings you say were slipping on your HW50. I have never heard of any BKL rings that are properly installed moving on any spring rifle, let alone one that is as smooth as the HW50. Something must be wrong with your installation. This is the very ring I would have recommended for the HW50 because of its precision. Are the rings really moving, or are you just concerned that they might? Because they won’t. Unless the rings are improperly installed or damaged in some way, they will hold onto the scope base of an HW50 so tight that any scope can be mounted securely.
If you want the absolute last word in precision scope rings take a look at the UTG P.O.I. rings. I showed them in detail in the report titled Optics test. I doubt there are rings on the market that are machined more precisely than these.
But here comes the real answer. Chasing specs like this to align a scope with a bore is a futile drill. Several years ago you read a lot about people finding the optical center of their scopes. There were even different techniques for doing it being widely discussed. You don’t read much about it these days because most shooters have discovered that it doesn’t make any difference. I remember 20 years ago when world-class field target competitor, Ray Apelles, told me that he had abandoned finding the optical center of his scopes. Instead, he just kept mounting and remounting his scopes and testing them after each tweak — by shooting them at different distances on the range and noting any shift in impact, side-to-side.
Ray told me that after he had optically centered a scope, none of his rifles would then shoot that scope correctly without some tweaking — FOR ALL THE REASONS MENTIONED AT THE BEGINNING OF THIS REPORT. It turns out that trying to fix the problem with specifications is a waste of time and money, because in the end all rifles have to be tweaked before their scopes and barrels can be correctly aligned.
Hunters get away with not doing this because the error rate (the amount the scope or bore is off the target) is usually very small. A bird doesn’t care if your pellet hits him a quarter-inch from the point of aim. A field target, on the other hand, can lock up and not fall if the miss is that great. You lose points. So those folks who really have to hit exactly where they aim find it worth their time to do the work of tweaking their scopes and mounts — at least the winners do.
What is meant by tweaking?
To tweak a scope or mount, you use all the tricks in the book — shims, adjustable scope mounts, two-piece rings that can be swapped, front and rear, and also turned around. Matt mentioned that his Chinese scope rings were off by 2 millimeters. Ray Apelles had mounts that varied by as little as the thickness of the vertical crosshair, depending on the way they were turned. That’s how a scope is tweaked.
And don’t forget to install a level on your rifle. That way you shoot every time with the crosshairs and bore in the same attitude. Three degrees off at 45 meters will drop your pellet by half an inch if your gun shoots at 850 f.p.s.
And — get ready for it — even with all that you do it never works out perfectly! Hans Apelles, Ray’s father, told me they could get their rifles hitting where their scopes indicated to within half a pellet diameter at 50 meters, but they had to account for that final bit with holdoff. That’s correct — men who have placed in the top ten in world competitions and click-adjust their scopes for every shot would also hold off by the tiniest margin, depending on the range and the target.
So, Matt, you can go one of two ways, but not both. You can either chase after rings that are perfectly aligned with the bore of your rifle when they are installed on the scope base of that rifle — which I said can never happen — or you can spend the time it takes to modify and fine-tune (tweak) your scope’s alignment until you have gotten it as close as possible. You may even need to try many different sets of rings until you find the set that works the best. Scope mounting like this doesn’t take minutes, it takes weeks. But it’s the only way to get the job done right.