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Installing Weaver Scope Bases on Contender Barrels

By: Mike Bellm

     Making a few checks along the way and taking necessary precautions installing Weaver scope bases will help keep the base secure and also help safeguard the screw holes from stripping out.

     Normal maximum thread contact is about 4 1/2 threads in Contender barrels. With less than 4 threads of contact, there is an increased risk of the screws stripping out due to over tightening, high recoil, or simply bumping or jarring the scope and mount. (Note that the TC "Duo Mount" is especially lacking in thread contact, so use caution with this design deficiency.) Holes should be tapped out with a "dead bottom" tap. This picks up about 1/2 turn of thread contact, but is best reserved to those experienced with tapping small blind holes. See the "More Advanced" comments below.

     The Weaver screws that come with the bases have a projection on the bottom end of the screw that is smaller and not threaded. This limits how far the screw can go into the hole and should be ground off. When I convert the bases to 6-screw, I cut the countersinks deeper and use different screws normally. I also have a special set up just for grinding screws to length with threads all the way to the end. An individual can do this himself, also, on a bench grinder or with a Dremel type hand grinder.

     When installing a Weaver base, be sure to tighten down each screw individually, then try to wiggle the base. If each individual screw does not hold the base down tight, the offending screws must be shortened so they do. Each screw tried should then be removed and examined for 1) deformation at the tip from bottoming out in the hole or threads not going far enough to the bottom of the hole and 2) marks on either the under side of the screw head or on the tapered part of the countersink in the base, indicating the screw is pulling down tight in the countersink.

     If there is deformation on the tip of the screw and the hole has been tapped all the way to the bottom, then grind some off of the tip a little at a time until the screw does pull down tight with no deformation of the tip.

     If you put a screw in the base, then hold the base up at eye level, sighting across the underside of the base, side to side, the tip of the screw should come just about flush with the bottom edges of the base. On barrels that have not had the holes tapped out all the way to the bottom, the screws may need to be very slightly above the bottom edge of the base. If you will then sight down the underside end to end and count the number of threads exposed on the bottom, you should come up with at least 4 threads. They are hard to see, yes, and a magnifying glass may be a must. You can also use a knife edge to follow the ridges of the threads down, following the crest and counting each thread as you go.

     Bolts and screws should always have a length of thread contact that is at least equal to their diameter, and if you will do some measuring, you will find that it takes this approx. 4 1/2 threads to achieve it on the 6x48 base screws. If the thread contact is less than this, they will be more prone to stripping out, and since the steel in the barrel is somewhat softer than the screws, it will most often be the barrel's internal threads that strip out. There are "saves" for stripped out holes using Brownell's .146x48 oversize 6x48 screws, but in these shallow holes increasing the diameter any at all is a step backwards, and fitting these screws which only come in about a 1/2" length can be a lot of work. Plus, stripped screw holes degrade a barrel, even when salvaged with the oversize 6x48s.

     Thread Lockers. I do use and recommend them. I use the "non-service re-moveable" grade of red Loctite. But most grades are ok except the bearing Loctite. However, with the cone shaped heads of the Weaver screws, with proper fitting of each screw, there is little likelihood a Weaver base will work loose.

     Which brings up a side note: Especially on barrels with brakes that produce fore and aft inertia forces on the base screws, I am totally, totally against fillister head (flat bottom) screws in bases, any brand or type of base. I go so far as to use Weaver screws in bases countersunk for fillister heads in order to get the security of the cone on the underside of the head to keep things centered and prevent movement of the base within the limits of the clearance that is always present around every screw in every base. I have from time to time also used fillister head screws in the Weaver type cone shaped countersinks... just so there is a cone somewhere to keep things pulled to center rather than having two flat surfaces, bottom of screw head and bottom of countersink, sliding against each other. Think bases don't slide under recoil? Think again. In many situations they don't, but in more extreme situations they definitely do. 4 puny screws, or even 6, simply do not have that much clamping force available.

     More Advanced: 6x48 taps are available from Brownell's, as are Weaver type screws with slotted, hex socket, and Torx heads.

     I'll not attempt to describe here how to grind a tap for dead bottom and must caution you that there is a relatively high risk of breaking off a tap while chasing the threads to the bottom of the hole. After many thousands of holes, I still break off taps every couple hundred holes or so. And, of course to save my bacon, I have pretty reliable ways of removing broken taps, but it is still no fun and puts a barrel at risk. So do use caution chasing threads to the bottoms of holes.

     If, or once, you have a 6x48 tap, one of the best ways to hold a screw for shortening it is to take a small strap of 1/8" thick steel and drill & tap a hole in it. Clamp the strap in a vise or use a "C" clamp to clamp it to a bench or other object to hold it steady, then screw each base screw into this hole one at a time. Then file or grind the end of the screw to length. I prefer simply filing them.

     Take a few strokes with the file and try the first screw in the base screwed down tight on the barrel. Remove the screw and make the observations above regarding deformation at the tip and indications of a firm seat in the countersink in the base. Once you get the right length, you can use the first screw's protrusion through the hole in the strap to gauge how much to take off the remaining screws. But because there will often be minor variations in the depths of the screw holes, do not assume that a given length will be right. The right way is to check each screw. It may take a little longer, but it will help guarantee that you are getting the maximum holding power of each screw.

     There is a right way and a wrong way to mount the Weaver bases, and unfortunately, Weaver is somewhat lax about the importance of full thread contact with their bases on Contender barrels. (Holes in Encore barrels are about 50% deeper, but still should be checked as above as a precaution.)

     While this article addresses Weaver bases in particular, it has of course parallel applications with all types of bases on Contenders.

- Mike Bellm

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This article reprinted with permission of Mike Bellm and
Warning: All technical data mentioned, especially handloading and bullet casting, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article or on this web site and over which The Los Angeles Silhouette Club (LASC), this web site or the author has no control. The above has no control over the condition of your firearms or your methods, components, tools, techniques or circumstances and disclaims all and any responsibility for any person using any data mentioned. Always consult recognized reloading manuals.