How to mount a scope: Part 1
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- The olden days
- What needs to be done
- Eliminate cant
- The tale
- More information
- The scope must be angled down
- Adjusting the scope too far right is also bad
- Not experts
- Position the eyepiece
- Adjustable scope mounts
- Is it enough?
- Points to remember
The olden days
When I started shooting in the 1950s, scopes were not that common, especially on airguns. I was as intrigued by them as anyone, believing that they increased the accuracy of whatever they were mounted on.
Well, they don’t. What they do is make it easier to shoot accurately with a given airgun or firearm. But they can only do it if they are mounted on the gun correctly and then sighted in properly. This series is dedicated to addressing all that is inherent in both mounting a scope correctly and then sighting it in properly.
What needs to be done
To properly mount a scope there are several things to consider. Here is a list.
- Secure mounting
- Proper eye relief
- Angle the scope so the elevation doesn’t have to rise above halfway
- Align the optical axis with the boreline
- Eliminate cant
I have written more than 30 reports on scope mounting since March 3, 2005, when this blog started. I will draw on them as I go, but I’m also going to break some new ground. Not that scope mounting has changed, but B.B. Pelletier has changed over the years. He has gained experience!
Canting is a subject all by itself. I won’t deal with it today.
To tell this story I will begin with a tale that happened to me a week ago. A friend from church told me the scope on his Gamo breakbarrel had broken — that the crosshairs are now sitting cockeyed
When the scope is still mounted tight on the air rifle but the reticle looks like this, the scope is broken.
In our second conversation he told me something more. He said his scope wouldn’t hold its zero. Right away I knew what the problem was. The scope’s elevation was adjusted too high and the erector tube return spring had relaxed, allowing the erector tube to bounce around as the rifle recoiled, and also vibrated, as I discussed yesterday.
The erector tube inside the main scope tube contains the crosshairs. When the scope is adjusted this tube is what moves. The crosshairs remain in a constant position and only move because the entire tube moves. The erector tube is also where the magnifying lenses are.
Where the reticle assembly is located in the tube makes a difference. If it is at the front of the tube it is called a first focal plane scope and because the erector tube is what magnifies the image, the reticle enlarges as the power is increased. If its located at the rear of the tube the reticle remains the same size regardless of magnification and that is called a second focal plane scope.
The scope must be angled down
What most rifle owners don’t know is the axis of their barrel points down. It is not aligned with their receivers, despite what they believe. This is so common on the AR-15 that special scope mounts with a down angle have been created. If you search the internet you’ll find AR-15 owners complaining that their expensive scopes and mounts won’t hold zero and they are getting lots of advice to change the scope and mounts. The problem is — the advice they are getting is all wrong. They have a barrel droop issue and have adjusted their scope’s elevation too high.
I have been at the range when a guy with an AR was sighting in. He had his elevation adjusted up past the 3/4 mark, which he had to, to get on target at 100 yards. The scope will never hold a zero when adjusted that high. He was shooting 4-inch groups before I had him adjust the scope down 60 clicks. Then his groups shrank to around 1.5 inches. Of course they were too low, but that can be fixed. Let’s get back to airguns.
When I examined the mount of my friend’s Gamo Whisper rifle I found a deep scratch along the top of the aluminum scope base on the rifle. Whoever mounted the scope, and I am guessing it was done at the factory, did not put the scope stop pin inside the hole in the scope base. Constant recoil and vibration caused the steel stop pin to slide along the top of the scope base and dig a deep furrow.
Whoever mounted the scope didn’t put the stop pin into the hole in the base. They just tightened it down as tight as it would go with the subsequent sliding along the base from recoil over the years.
This scope stop pin on the bottom of the one-piece scope mount of the Gamo Whisper should have been placed in the scope base hole, not tightened against the top of the base.
Adjusting the scope too far right is also bad
Adjusting the scope’s elevation too high is bad and the same holds for adjusting the scope too far to the right. The erector tube spring relaxes and the tube starts moving under recoil. But too far to the right is rare because it’s easy to see, where the droop is harder to detect.
This is where buyers sometimes miss the boat. They think that if the company that made the rifle also mounts the scope it has to be done right. They don’t appreciate what goes on in those companies — that someone with little or no training is given the task of installing scopes on rifles. They certainly don’t then shoot each rifle to test their work. Some will mount them right and others won’t.
Now, if a retailer like Pyramyd Air mounts a scope they do it right because it’s their name on the line. I used to scope rifles for customers when I worked at AirForce Airguns. I did take the time to zero each of them, which is how I came up with the 10-foot sight-in process that I wrote about. And I gave the 5-shot group I shot to the customer, so they knew how well their rifle could do. But that kind of service is the exception rather than the rule. What we see here — the mis-mounted scope mount and no regard for barrel droop is more common.
Position the eyepiece
Another complaint my friend from church had was the eyepiece of his scope was positioned too far forward. All he saw was a small image instead of the full image the scope is supposed to show. So I also took care of that when I mounted his new scope.
Adjustable scope mounts
The best solution to a rifle that droops is to mount the scope on an adjustable mount. It’s expensive but my vote goes to the Sportsmatch scope mount that can be adjusted while the scope is on the rifle. But a guy spending $275 on a breakbarrel bundle isn’t going to pop for $150 more just for a scope mount. So my less expensive solution is to shim the scope under the rear ring.
There is the shim I put under the scope on the rear ring. It’s a cut-up credit card.
And this is what that one shim did. The scope base is level. The scope is slanted on a downward angle that’s visible, relative to the scope base.
When you tighten the ring caps don’t tighten them too tight or the scope tube will bend. When it’s shimmed it’s no longer cradled in the ring that’s designed for it. The new angle will help keep it from moving in the rings during recoil, so the caps don’t have to be as tight.
Is it enough?
Is what I did enough? We won’t know until the rifle is sighted in and tested. That job falls to its owner, though I stand ready to help if he needs anything further.
Points to remember
- Most rifles shoot down and need their scopes mounted to compensate for it.
- Scope rings need to be mounted to the scope base correctly if they are to do their job.
- If a scope’s elevation is adjusted too high the scope may not hold a zero. Same if it’s adjusted too far to the right.
- Don’t over-tighten the scope caps when the scope is shimmed.
That’s it for this report. If I’ve left something out or not explained it well enough, let me know.