Scope Base Screws — An Old Standard

Guns Magazine, June, 2001 by Dave Anderson

Long ago, some unknown gunsmith drilled and tapped a rifle receiver ring to attach a sight base. The machine screw size he chose was 6-48. Since then, the 6-48 base screw has become the industry standard.

Why this oddball size was chosen is a mystery. Custom gunmaker Ed Brown commented that if you went into a machine shop to buy a 6-48 tap, they wouldn't have it. The only purpose for which 6-48 screws are used is to attach sights to rifles.

A #6 screw has a diameter of 0.138". The second number denotes threads per inch (tpi), so a 6-48 screw is a #6 with 48 tpi. Industry standards for #6 screws are coarse (32 tpi), fine (40 tpi), and special (36 tpi). The 6-48 size is an oddball.

Even the description of the Brownells Screw Chek'r (a device for identifying screw size) notes that it "covers all standard sizes, but not 6-48 or the other bastard sizes." Of course, Brownells supplies 6-48 taps and screws to the gunsmithing trade, and scope base manufacturers supply screws with their bases. It's not like we're going to run out. So what's the problem?

It used to be that heavy target scopes were mounted mostly on light-recoiling rifles. Powerful rifles either used iron sights or light 2 1/2x or 4x scopes -- and probably weren't fired often.

Today's shooters want light rifles, powerful cartridges, and bulky, heavy scopes. Keeping over 20 ounces of scope attached to a .338 Ultramag rifle is asking a lot of four small screws. Some gunmakers feel that the #6 size isn't adequate for current needs, and that 8-40 (0.164" diameter, 40 tpi) should become the new standard.

Ed Brown's custom 702 action comes drilled and tapped to accept 8-40 screws, as do other custom actions such as Nesika and Geske. Some custom gunmakers routinely convert factory actions to accept 8-40 screws by redrilling and retapping the holes. Brown believes it's only a matter of time before the major gun manufacturers switch to the larger screws.

What about the millions of rifles already out there? Properly attached, 6-48 screws do quite well. They are specially hardened and heat-treated, and they thread into the tough steel of the receiver. Most makers recommend they be tightened to 20 inch-lbs., which is not that easy to achieve with slotted or Allen-head screws.

Screws with Torx heads are easier to tighten. These come standard on some new bases, and are available from several base manufacturers and from Brownells. The bases, screws and receiver should be degreased with alcohol, and a thread-locking compound should be applied to the screws. Cinched up tight, they are adequately strong for most purposes.

For extra security with heavy scopes or powerful rifles, Brownells sells an 8-40 base conversion kit to assist gunsmiths in adapting factory actions. This is not a job for the home hobbyist. Drilling and tapping the tough steel of a receiver, while keeping the holes straight and aligned, is a task for a good machinist.