Mounting Mishaps
Joining rifle and scope imposes hazards. Most rifles are drilled and tapped for 6-48 screws, “6” denoting shank size (albeit, like #6 shot, not the measure of diameter). The “48” means there are 48 threads per inch (tpi). The higher the tpi number, the stronger the screw.
I’m told 6-48s hailed from Winchester for sight and scope mounting. In a perfect world 6-48s would withstand the stiffest recoil — but surface irregularities exist. Mounts don’t all hug receivers like skin on a peach. Imperfect matches in contour reduce contact whose friction helps screws bear shock.
Drills and taps must allow for dimensional variation in screws. Hole depth is another issue. As a 6-48 screw is 0.130″ in diameter; a hole just 0.130″ deep gives it maximum resistance to failure. But the bridge on one of my rifles is 0.090″ thick — 0.040″ shy of this depth. A similar shortcoming exists where bases with countersunk holes leave wafer-thin webs under the screw heads. If head and shank are smaller than base holes for them, the underside of the head and a sliver of base must stand alone against the slam of recoil.
Growing use by manufacturers of 8-40 screws helps mount bases stay put and reduces the jarring effect on the scope of even slight shifts during recoil. Keen to replace 6-48s with 8-40s? Engage a careful gunsmith — more than one weekend tinkerer drilled the mount base through to the chamber. Upon firing, the cartridge case ruptured, launching the screw and it punched a neat hole in the scope tube.
Shooters commonly over-tighten screws — use a torque wrench! Gunsmith catalogs have affordable, firearms-specific torque wrenches. They help you snug screws uniformly and brake your zeal before you strip threads, disfigure the screw head or lose control of the bit, scarring mount or rifle.
How much torque? Depends on the screw and what is absorbing the pressure. Bases of scope mounts should be tighter to the receiver than rings around the scope tube.
Cinching base to rifle, I bring 6-48 screws to 25 inch/pounds — more than advised by Leupold and Badger but in line with recommendations by Nightforce and Warne. Leupold suggests snugging 8-40s to 28 inch/pounds, or 20% tighter than 6-48s.
Torque recommendations for ring-to-base attachment vary from 14 to 65 inch/pounds depending on mount design. Ring diameter can make a difference too. Weaver advises 30 inch/pounds for crossbolts attaching 1″ rings, but hikes the number for 30mm rings and again for 34mm. Talley recommends 30 inch/pounds for the bottom screw gripping the base of its vertically split rings, 20 for the top. For 10-32 screws in QD mounts, the numbers are 35 at bottom, 20 at top. According to one source, the crossbolt nut cinching rings to Picatinny rails merit 65 inch/pounds.
Joining the halves of horizontally split rings, you needn’t exceed 20 inch/pounds. In fact, Vortex sends a note with its competition scopes, warning shooters not to exceed 18. Leupold and Badger advise 15 to 18. Snug these as you would lug nuts on a truck wheel: alternately, diagonally and, as you approach “tight enough,” incrementally.
