Mounting a Rifle Scope? Avoid These 5 Mistakes

Mounting a rifle scope is well within the capabilities of anyone who can use a screwdriver without injuring themselves. However, there are some careful steps which must be followed and some common mistakes to avoid. We’re not going to discuss how to mount an optic to a rifle receiver here; you’ll have to go somewhere else for that. Instead, we’ll assume some knowledge of scope mounting processes and lingo, limiting ourselves to discussing common scope mounting mistakes, all of which I’ve made myself, and some of which I’ve seen made by “professionals.”

Using the Wrong Rifle Scope Base Screw Length

Of course, the mounting process starts with firmly attaching the base to the rifle. If that’s not done properly, every other scope mounting step is wasted. A common mistake when mounting the base is using screws that are too long and interfere with the bolt’s travel. A novice shooter once brought me his new rifle with a scope which had been freshly mounted by a person at a retailer’s “gun-counter.” But ever since the scope was mounted, the bolt wouldn’t open. Yes, one of the base screws was too long and had locked up the bolt. Obviously, the scope was mounted with the action closed and was never opened again before I delivered it to the customer.

Incorrect Rifle Scope Ring and Base Screw Torque

I’ve been accused more than once of having a screw loose, and sometimes it’s been the optic I just mounted on a rifle. With as many as 16 screws involved in attaching a scope to a rifle, it’s easy to mess up. All these screws have a torque value to which they should be tightened, with those values varying widely.

My most common mistake is not tightening the screws that attach the rings to the base. I normally tighten these only partially until I’ve adjusted the scope for cant and eye relief, then tighten the ring screws and promptly forget the ring-to-base screws. The solution is a quality torque driver and checking all screws when the job is finished.

Improper Rifle Scope Positioning and Eye Relief

Failing to set the scope’s ocular lens at a proper distance from where the shooter’s eye will be located is another common mistake. Not only do the physical properties of the shooter’s build affect where the scope should be mounted, fore and aft, but so does the most likely position from which the rifle will be shot.

Obviously, a shorter shooter, including one with a short neck, will need a riflescope set further aft than a tall, lanky shooter. This adjustment is typically accounted for. However, what’s usually missed is checking the positioning from an expected shooting position. For example, what will be experienced as the correct scope position when shooting from a standing position will inevitably be too far forward when the rifle is shot from a sitting position. Whenever possible, check scope positioning by assuming the position from which the rifle will most likely be shot.

Rifle Scope Cant and Reticle Alignment Errors

To deliver on their tracking capabilities, modern rifle scopes need to be oriented perfectly in their vertical/horizontal planes. Failing to do this results in tracking errors, where a vertical adjustment also introduces an unwanted windage adjustment. Fortunately, many tools help keep things square and level. Simple tools index off the scope base or the top of the vertical adjustment turret, while more elaborate systems use a plumb line with a system of lights and lenses. If perfection is required in tracking, a tall target test is still necessary after these tools have been applied in the shop. It remains the best way to determine whether a scope provides true vertical tracking.

Incorrect Scope Ring Height and Clearance

And finally, sometimes a scope doesn’t physically fit on a rifle. I’ve seen this when an optic’s objective lens makes hard contact with the barrel, and the owner is completely unaware it was installed that way. Clearance of the objective lens is an important check before you do the final torquing of the ring screws. Then check again afterward.

Improper fit can also show up at the other end of the scope when the bolt handle doesn’t have sufficient clearance to operate freely. Owners will typically catch this one because of the pinched skin and bruised knuckles it causes. With the current trend of building actions with bolt lifts under 90 degrees, this is becoming less common. The solution to both problems is usually as simple as using slightly higher rings.

How to Avoid Common Rifle Scope Mounting Mistakes

This list is the byproduct of mounting hundreds of optics to rifles. While mounting a scope is easy, doing it right takes care, patience, and a few dedicated tools. Just don’t assume these are the only ways to mess up. There are more. These are only the most common ways.

About The Author

Al Voth calls himself a “student of the gun.” Retired from a 35-year career in law enforcement, including nine years on an Emergency Response Team, he now works as an editor, freelance writer, and photographer, while staying active as a consultant in the field he most recently left behind—forensic firearm examination. He is a court-qualified expert in that forensic discipline, having worked in that capacity in three countries. These days, when he’s not working, you’ll likely find him hunting varmints and predators (the 4-legged variety).