How to mount a scope: Part 3
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- The Design an Airgun contest
- Air gun?
- How to enter
- I lost one entry
- The Godfather’s Gold Gun drawing˜
- On to today’s report on cant
- Canting is not part of scope mounting
- What is cant?
- The cant test
- What cant does
- Things that affect cant
- What canting can do
- When precision is a must
- How to eliminate cant
- High scopes
- Where the level goes
The Design an Airgun contest
Apparently it took a while for many of you to realize this Design an Airgun contest was happening, so I’m extending the deadline to Friday, October 16. I’m challenging you to design an airgun that we readers can build!
I’m guessing it will be a BB gun, but it doesn’t have to be. It doesn’t even have to be a gun, as long as it shoots something at a target. If it is a gun I’m guessing it will be a smoothbore, but again, it doesn’t have to be.
When I say build an airgun, it doesn’t have to work with compressed air. The Daisy 179 pistol is considered an airgun, but in reality it is a catapult gun. And spring-piston guns don’t have compressed air until the instant they fire.
How to enter
You can enter by writing your design as a comment. Attaching pictures helps me a lot. One reader has already entered by submitting a You Tube video. Or you can email me at [email protected] I’m wanting to show all of you how the winning design works in a special report, so any help you can give me is appreciated.
I lost one entry
To the reader who submitted two designs with photos to me via email several weeks ago — please send them again. I have lost them. I remember making a file of your entire submission, but after three days of searching I cannot find it. You were the first person to submit an entry and you explained it well enough that people could follow along and build it themselves.
The prize for the winning entry is the odd gun I have shown you many times. I called it the American Zimmerstutzen. I have to limit the contest to US readers because of the problems of shipping the prize.
This is the prize for the best airgun we readers can make. It’s an intricate homemade weapon that I think is the perfect prize for this contest!
The Godfather’s Gold Gun drawing
One day in October I will choose a winner from all the readers who comment. Whether you comment once or many times makes no difference. Your comment enters you into the drawing for this air pistol. I will not announce the day I select until the month is over. Once again, this is limited to US readers.
The Godfather’s Gold Gun will belong to one lucky blog reader after October is over.
On to today’s report on cant
Today we look at cant, as in tipping the rifle and scope while shooting. I’m going to sum up today’s report in a single word — consistency. Because that is what we are after. The report will explain.
Canting is not part of scope mounting
We are concerned with canting on every shot we take. It’s not something that gets addressed while mounting a scope — BUT!!!! So many shooters associate canting with scope mounting that we need to discuss it now. Part 2 was the last part of scope mounting, per se.
What is cant?
Cant means a prominent angle or tilting, as defined by dictionary.com. But in shooting it means something far more devious. Yes, it is a tilt, but it is also an inconsistent tilt, and it’s that inconsistency that creates the problem.
The cant test
When I wrote The Airgun Letter I did a cant test with my subscribers. I asked them to shoot three groups of 10 shots at 50 yards. One group was to be a regular group where they were to zero their scopes to hit the center of the target. I sent out special targets that had bolded lines for the shooters to cover with their reticles. One set of lines was tilted 20 degrees to the left, one set was aligned correctly and one set was tilted 20 degree to the right. If they covered the lines with their reticle they would shoot one group straight, a second group canted 20 degrees to the left and a third group canted 20 degrees to the right.
The results of the test were dramatic and all were the same. Everyone was sighted to hit the center of the target at 50 yards. When they canted 20 degrees to the left their 10-shot group landed low and to the left. When they shot straight on their groups hit the center of the target. When they canted 20 degrees to the right their groups landed low and to the right. Let’s look at the results from four shooters.
These are the results of 4 different shooters shooting 10 shots at each of three targets at 50 yards. Targets on the left were shot with the scope canted 20 degrees to the left. Same on the right. Center targets were shot with the scope level. We used the heavy lines on each target to align our reticle.
What cant does
When a scope cants away from the orientation at which it was sighted-in (notice, I did not say from where it was level), the pellet impacts in the same direction as the cant. A right cant produces a pellet impact that’s more to the right than it should be. The pellet also drops away from level at the impact distance. The next drawing shows this very clearly.
The heavy curved line represents where pellet will impact if the rifle is canted from its sight-in position. The farther the distance from the shooter to the target, the more pronounced the curve.
Things that affect cant
Distance — the farther out you go the more affect cant will have.
Height of scope — The higher the scope is (farther from the bore axis) the more affect cant has.
Velocity — the higher the velocity the less affect cant has. If you look at the picture of the cant test you’ll see that some pellets didn’t drop as far as others. That was due to their velocity.
I made this test exaggerated to show the effects of cant dramatically. Most shooters would never cant by as much as 5 degrees, let alone 20. There are, however, some things outside the shooter’s control that will also enter into the discussion.
When I worked at Frontier Village amusement park in the 1960s, we had an attraction called El Sito Mysterio. It was patterned after an attraction in the nearby Santa Cruz mountains called the Mystery Spot, where gravity was supposed to be all confused. El Sito at the Village was a small shack that had several interesting illusions. My favorite was the bowling ball that rolled uphill. I worked there and gave the pitch and even I was fooled! So terrain and visual cues can fool anyone into making a mistake.
What canting can do
Even a one degree cant has an effect on shot placement. Let’s now look at two groups that were fired with and without cant.
This drawing is representative of how cant affects a group. This is why it isn’t that noticeable. It just looks like poor accuracy. It’s easier to see at 50 yards than at 10.
When precision is a must
Cant doesn’t always matter. To a hunter shooting in the woods, a fast rifle with a quick second shot is more important. The squirrel doesn’t care if the pellet lands 1/4-inch from the intended impact point. But to a varmint hunter busting prairie dogs at 300 yards, cant is very important.
Field target shooters are quite aware of the effects of cant because when they hit their target one pellet’s diameter off their intended impact point they may hit the side of the kill zone and lock the target in the upright position. Canting costs them points, and they are already fighting their own body’s stability. They don’t need a second problem.
I said at the start of this report that consistency was the solution to canting. Now I’ll explain why that is. When you shoot, the pellet leaves the muzzle for the target — hopefully with the muzzle in the same place each time. By canting or tilting your gun, you move the scope off target and then you have to move the gun to get it back on. When you do, the muzzle is in a slightly different place than before. It could be less than one-tenth-inch or 2.54 mm, but it is different and, if the gun is accurate, the pellet will fly to a different place. And that does affect accuracy in a most insidious way, as you see in the group above.
How to eliminate cant
Cant is eliminated by leveling the rifle before each shot. This is done with an accessory called a scope level. It doesn’t matter where the reticle is — so long as it is always in the same place when the rifle fires. If the rifle is accurate and if it shoots to the same place every time, the group will be small and exactly where the shooter wants it.
Let me show you a short test I did that shows the importance of this. The test is straightforward. First, I seasoned the bore with several shots. Then, I fired a group of 10 shots at 25 yards with the Whiscombe rifle canted to the right for 5 shots and to the left for 5 shots. The cant was controlled by the position of the bubble in the level, and I stopped tilting the rifle the moment the bubble came to the end of its travel. Obviously, there’s some error in this, as the bubble level is not a precision instrument, but I think you’ll get the idea.
The Whiscombe was canted to the left until the bubble came to the end of its travel on the right side, as shown above. Five shots were fired at the target with the crosshairs on the center of the bull at 25 yards.
The Whiscombe was then canted to the right until the bubble came to a stop on the left side, as shown above. Five more shots were fired at the same target, just as before.
And this is the result. It’s impossible to tell that canting opened this group.
Same rifle, same distance, same pellet, only this time the rifle was leveled for each shot.
Scopes that are mounted high above the boreline are influenced by cant the most. This is why many shooters want their scopes mounted as low as possible. Repeaters with magazines that stick out above the receiver mitigate against this, of course.
Where the level goes
Mount the level on the air rifle where it best fits how you shoot. When I use a separate bubble level I mount it sticking out to the left side of the rifle. Before each shot I look at the bubble and make any corrections that are necessary. But best of all is when the bubble is built right into the scope — like it is on the UTG Bubble Leveler scope! Then I don’t have to take my eye off the target.
Canting isn’t a part of mounting a scope, but if the scope is mounted correctly then the rifle can be leveled for each shot afterward with assurance that it will do its best.
We are still not finished with this series. The next report will be about optically centering the scope and centering the reticle mechanically. I hope you are getting something out of this.