Barrel droop is a condition that affects scope mounting in a major way. You might think that a rifles barrel points straight out from the gun it’s mounted on, but most of the time that’s not the case. This downward angle makes mounting and sighting in much more difficult.
When the manufacturer installs a barrel there is no guarantee that it will point straight ahead and this applies to both break and fixed barrels. These barrels can all be out of alignment and most of them usually are.
Unless the manufacturer takes special steps that require careful measurement and hand machining, the barrel will almost never point straight out from the rifles action. Thus, when you mount a scope on the action there is no guarantee that it will be set in the exact same direction as where the barrel is pointing.
This is true for air guns as well as firearms. Those special extra steps usually add hundreds of dollars to the manufacturing process so you can be sure they’re not done on lower cost rifles.
Because the barrel and scope point in slightly different directions, it’s much more difficult to get the scope aligned with the pellets exact direction. While the barrel can point in any direction, remember most of the time it points slightly downward.
When a scope is mounted on a rifle that has a downward angle barrel, the shot will strike below the aim point. This may make it next to impossible to adjust the scope enough with its internal adjustments to get the aim point and the shot to coincide.
What is the cause of most accuracy problems with scoped air rifles?
The erector tube which is inside the outer scope tube is under spring tension that opposes the scope knob adjustments. When the adjustment knobs are turned, they move the erector tube either against or away from the tensioning spring.
To get the pellet to strike where the crosshairs are or the scope are set, the shooter has to adjust the scopes internal vertical elevation knob upward.
There’s a point where the tensioning spring which presses against the erector tube fully relaxes. When that happens, the erector tube is free to move around from the vibration of each shot and the scope will no longer hold a zero. At this point you may begin to see accuracy issues as a result of barrel droop.
Compensating for barrel droop
To compensate for barrel droop you have to raise the rear of the scope slightly when it’s mounted so that it also points downward. This is done by a process known as shimming. A shim is a thin piece of flat material like the plastic from a soda bottle or something as simple as a business card. The shim material should be cut to the same size as the width of the scope ring and placed inside the rear ring under the scope tube.
It raises the rear of the scope a few thousands of an inch which is all you need when the barrel droop isn’t too great.
What you’re trying to do here is get the scope angled downward. When the scope is zeroed, the vertical adjustment knob is close to the center of its adjustment range. When the knob is adjusted ¾| towards the maximum, the erector tube spring is very close to being relaxed. This is when the problems of maintaining a zero begin.
If the erector tube spring gets too relaxed, the erector tube will jump around with every shot; this allows the aim point to shift. It’s important to use only thin shims for this kind of scope installation because too much shimming causes the scope tube to bend when the scope caps are tightened.
If the scope has to be angled down more than what a few thin shims will do you need to use an adjustable scope mount to get more height without bending the scope tube.
The object of shimming a scope is to make sure the vertical adjustment knob is far below the three quarters elevation mark once it’s mounted. You can also feel when the erector tube starts to lose tension; the scopes will become mushier to turn and the clicks become less well-defined. This effect is found more in lower-priced scopes and varies from brand to brand.
In most cases, it’s the fastest way to recognize when the vertical elevation is too high. If you try to zero a scope that has a relaxed vertical erector spring, the zero point will shift at random.
You may be able to sight in the scope, but at some point the groups will shift to another place. This will continue until the erector tube spring is put under sufficient tension to hold the tube in place shot after shot.
Just as the vertical adjustment can allow the erector tube spring to go slack, the horizontal adjustment can do the same thing. This happens when it is adjusted too far to the right, causing the erector tube spring to relax. Remember; if you are having major alignment problems, you will need an adjustable scope mount.
In conclusion, compensating for barrel droop can be done either through the use of shims or by purchasing an adjustable scope mount. If you want to be cost-effective go for the shims, while an adjustable scope mount is ideal if you’re looking for convenience.