Precision scope mounting

It’s now or never

With rifle season just around the corner, the calls and inquiries start each year about the proper way to mount a scope on a hunting rifle. The operative term here is proper. If you buy a new rifle with a scope and allow the store “gunsmith” to mount the scope, it had better be somebody you really trust. This is even more important if they also perform a bore sighting for the new scope on the rifle.

A familiar story

An old neighbor of mine called before the deer season last year saying he could not get his new deer rifle to hit “squat” at the rifle range at 200 yards. That is the range the “expert” at the store recommended he start shooting. I immediately knew he was headed for trouble.

I told Richard to bring the rifle by the house, and I’d take a look at it. Out of the case I examined the excellent Sako rifle he had bought and the very expensive scope on it. It was easily a $2,000-plus rig. I also noticed the front bell housing of the objective scope lens was mounted slap up against the front ring with no spacing. I try to avoid this arrangement at all costs.

With a quick shoulder mount against the white wall in my office, I took note the crosshairs were not aligned square with the horizon. When I asked Richard about his sight plane through the scope, he agreed the crosshairs were canted. This is early evidence of a bad scope mount.

Try, try again

So, I got out the proper tools, locked down the rifle in a gunsmithing rack, and preceded to remount the scope. In the process, I double checked the scope mount screws on the rifle after taking the scope out of the rings. They needed a couple extra turns. Had this been my rifle, those mounts would have been secured in place with a drop of Locktite screw adhesive.

Long story short, I got the new scope back in the mounts and rings correctly, and aligned it without cant to suit the owner. I also properly adjusted the eye relief (distance of the rear scope objective to the shooter’s eye) as well as the scope focus. The owner knew nothing of these adjustments, and apparently neither did the salesman at the big box store.

I bore sighted the scope, and recommended the shooter begin sight-in shooting at the range of 50 yards. Once he could see that it printed on paper, he could start to make the correct scope adjustments to fine tune his desired point of impact at 100 yards. I recommended the standard of 3-inches high at 100 for his 300 Winchester Magnum.

Richard reported back that within six rounds of firing the rifle it was hitting the desired target sight. I told him if he could repeat this a couple more times before the start of the gun season, he ought to be set up for hunting.

Additional scope-mounting tips

“Before I start the process of mounting a scope on a rifle, I like to begin by degreasing everything connected with the process using rubbing alcohol,” says Jim Jackson, chief gunsmith for World Wide Firearms in Gulfport. “This means I wipe down all the mounting surfaces on the rifle, clean the mounts, mount screws, rings and ring screws.

“I soak a cotton tip in alcohol and twist it down into the pre-drilled mounting holes in the rifle’s action to clean grease and oil out of those holes. The alcohol will evaporate quickly leaving a clean, dry surface. This will make a strong seal when all the screws are tightened down.

“Before I turn the mount screws into the rifle, I will put a drop of gun screw glue on the threads, but just a dab. This will seal the screw threads into the mounting holes, but will allow their removal at a later time. I install the rings according to manufacturer’s directions as different styles and brands of rings require different installation procedures to be done properly.

“Before I set the scope into the rings, I wipe it down as well. I sometimes use a piece of 400-grit sandpaper to quickly sand down the inside of the scope rings to create a clean surface offering a little grip. This will help stabilize the scope in the rings to keep them from creeping in the rings once mounted.

“Setting the cant on the crosshairs can be a pain. There are several aids on the market to help with this. Some employ a bubble level which seems to work well. Be sure to shoulder mount the gun carefully just to get a bird’s eye view to make sure the crosshairs are square to the horizon. Check the eye relief again before the final tightening process. Focus it when done.”

Jim also advises when you crank down on all mounting screws to go easy.

“Rotate the tightening process like rotating tires to balance the pressure on all screws,” he said. “Watch the gap between the ring top and bottom front and rear to make them as close to the same on both sides of the ring as possible. Do not overtighten.

“When complete, give the metal surfaces a light wipe down with gun oil, then clean the lenses with the appropriate cleaner and lens tissue.”

Bore sighting is the final step. It can be tricky. Use a commercial bore-sighting device and follow the guide carefully. You are essentially trying to “zero” the crosshairs on the scope to the grid inside the bore sight optical unit. Center both the up/down (elevation) crosshair with the right/left (windage) matched to the center of the grid.

Bore-sighted does not equal range-sighted. This is where a lot of hunters end up missing the big one. If you rely 100 percent on a mechanical bore sighting at the gun counter, then you are foolish. To do it right, every hunting rifle and the owner/user/shooter has to shoot the rifle from a stable rest at a known range until it hits the bull’s-eye or 3 inches high over it. Then and only then is it sighted in and ready to go. Check it again every year or if the rifle gets banged or dropped.