Tapping a S&W 686 for scope mount

smootz · Sep 23, 2012

I typically post in the General section as I am not a gunsmith.

I just bought a Leupold base and rings for my 686. It is an older model that was not tapped from the factory. After viewing the sorry equipment that my dealer intended to use on my gun I decided that I should maybe try this myself.

DSCF0630 by jsmootz, on Flickr

The gun is SS and will require 3 holes, two are centered in the back strap and a third just ahead of the hole that mounted the open sight. The front hole will be "blind".

The provided screws mic .136 and are approx 46 - 48 pitch best I can tell. I'll need a bigger magnifying glass to determine for sure.

I have a couple of regular drill presses and a Bridgeport Vertical Mill. Can anyone give me advise on procedure and fixturing so I don't ruin this gun?

Thanks, SCOTTIE

RWO · Sep 23, 2012

The screws are 6-48. Remove the cylinder and place the barrel in a padded mill vise on your BP. Let the frame overhang the table. Put a wood block under the butt if you like. but you can grip the barrel tightly with out damaging it. Make sure the barrel is square with the spindle axis. Drill and tap each hole using the drill chuck to insure perfect alignment . The tap is a gunsmith's special(Brownells) but you can buy them at MSC. The drill is a #31.

RWO

smootz · Sep 24, 2012

Thanks. Is the back strap hardened? Does it require any special alloy drill and tap?

BWE Firearms · Sep 24, 2012

The top strap is not that hard so you can use a standard #31 drill.

speerchucker30x3 · Sep 24, 2012

Make sure that you run a dial indicator down the side of the inside of the mortice for the rear sight before you poke holes in it. The bottom of the base fits snugly into the mortice with some base's and the barrel and receivers on those guns often point different directions. If its to late to do that, machine the base, not the mortice for the rear sight so that your holes line up. If you try to force the holes to line up by tightening the base into the mortice while its misaligned it often screws up the clean lines where the sight fits in. Its been done to death.

When I do them, if the base fits snug to the mortice I tap the base into place and then super glue it for good measure and find the center of each hole in the base with a center finder and drill them with the base in place, one at a time. It adds 20 minutes to the job but it comes out perfect every time

smootz · Sep 24, 2012

The base fits into the mortice but not really tight enough to consider it a "tap in". I had intended to clamp the base to locate the first hole and then put a screw in. I like your super glue idea better.

One hole is ahead of the top strap and will be blind above the barrel. Will that cause much grief? Will it require an additional bottom tap?

The other two holes are centered in the strap. It is tempting drill them only. This is a 357 not a 44. Would two holes be enough?

speerchucker30x3 · Sep 24, 2012

I often clamp and glue them on with accraglass before I drill and tap them and then leave the glue bond intact. If there were 10 holes, I would use them all. Attaching heavy objects to things that try to jump into the next county every time you scare them is like trying to stuff a cat into a shoe box. It seams simple until it gets away on you a few times. I always bottom tap shallow holes. Personally, I would use a center finder on each hole. Especially if you hold the barrel in the vice as the barrel and receiver are often going in different directions.

frank46 · Sep 25, 2012

Sir, if you look at the front hole that is partially drilled its done like that for a reason. Most likely the rear end of the barrel is directly where that hole would be if fully drilled all the way through. Far as the other two holes are concerned if the revolver were mine, I'd drill completely all the way through and then tap. Much easier to tap a through hole than a blind one. If the threaded holes on the underside of the top strap bother you you could most likely get stainless steel plug screws from Brownells and just screw them in. Hope this helps. Frank

smootz · Oct 31, 2012

I finally gathered the guts to tap the gun. Deer season is approaching and I figured I better get it done. The process went much smoother than expected.

Now I need some advise on attaching the rings. It appears that they just twist into the base but it will require considerable force to make that happen.

Should twisting them in require a wrench or tool? Do I attach them to the base before or after attaching the base to the gun?

Any advise appreciated. I hate to travel to a gun shop to watch someone just pop this together, look at me like I am stupid, and charge me 20-30 bucks.

Sorry for all the questions. I just don't want to screw this gun up.

DSCF0769 by jsmootz, on Flickr

speerchucker30x3 · Oct 31, 2012

Very simple. Clamp each ring, one at a time to a piece of 1 inch aluminum or steel bar and turn them in (they should turn in uncomfortably hard). After turning in both of the rings you can lay the bar in the lower halves and check for alignment and adjust as necessary. Then clamp in the scope.

smootz · Oct 31, 2012

Should the base be attached to the gun before twisting in the rings? (or does it matter?)

speerchucker30x3 · Oct 31, 2012

smootz said:
Should the base be attached to the gun before twisting in the rings? (or does it matter?)

Either or, makes no difference except in some cases the rings may cover the base screws so attached is best.

smootz · Oct 31, 2012

Thanks - I owe you a deer burger.

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