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Thread: Drilling & Tapping a Barrel for a scope mount

  1. #21
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    Chill Wills's Avatar
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    Good info here.
    I like Bent Ramrod's post.
    I would add, when paying a gunsmith a lot per hole, some of that cost of getting it right is he will likely use his mill to set up and drill precise holes and that takes some time to set up.
    Faster is to just eyeball the top center and punch the spot that looks good. There are all kinds of "gunsmiths" out there. Choose your smith well by asking question about how he will do it.

    For us shade tree gunsmiths, I would add:
    Use a drill press and start with a center drill AFTER you are damn happy with your lay-out (all holes) are punched. Check it twice and then check it again.

    Use the quill stop on the drill press.

    Did I mention, the start the hole with the correct center drill? That will keep the small drill from wandering off to parts unknown.

    If you can retain the set-up, go ahead and tap each hole as you go using the drill press to hold the tap center over the hole.
    Go slow and feel everything as you do it.

    Last, and this was touched on in an earlier post. Go to the rehearsal. Try this out a few times on scrap. Get it right. You will be happy you did.
    Last edited by Chill Wills; 05-01-2020 at 11:33 AM.
    Chill Wills

  2. #22
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    If you are only going to drill and tap to 1/8" because of your screw length, I will gladly send you 3 or 4 screws that are double or triple that length so you can get your base more properly and securely mounted. I'll even send you the taps to allow you to do the job right! Send me a PM if you are interested. Consider this a very early Christmas present! This is one of those situations where you either do it right, or you don't do it at all.

  3. #23
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    The Williams Scope Mount Fixture uses drill bushing. https://www.midwayusa.com/product/1007371387

    When using a fixture with drill bushings center punching is not needed and it may be counterproductive.

    Same for traditional center drills. They generally don't have the proper body diameter and are too short. A spot drill is sometimes used for some applications but that is very rare. Normally it's when a drill enters a casting or forging at a steep angle. https://www.mscdirect.com/browse/tnp...%20-%20Product

    Drill bushings explanation https://en.wikipedia.org/wiki/Drill_bushing

    The fixture is designed to have the bottom of the drill bushing touching or very close to touching and the hardened bushing should be less than a 1/2 thousands over the drill size so the drill won't wander even on a curved surface.

    I have never used the Williams fixture but if it's setup properly is will have two sets of bushings. One for the drill diameter and one for the tap diameter.

    https://williamsgunsight.com/product...drill-fixture/

    WILLIAMS SCOPE MOUNT DRILL FIXTURE
    The Williams Scope Mount Drill Fixture is designed for drilling almost any receiver between the diameters of .630″ to 1.535″; round, flat, or octagonal. With a hole spacing choice of .500″, .605″, or .860″, you can install almost any scope mount base in the industry.

    The “scissors” action of the fixture will automatically center itself. The fixture comes complete with one each #31 and #28 hardened steel lock bushings for drilling #31 (6-48) and #28 (8-40) thread sizes. When locked into position, the lock bushings will not spin in the fixture, avoiding scratches around the drilled holes. After #31 holes are drilled, you can use the #28 bushing as a tap guide for 6-48 taps. You will have fewer broken taps and perfectly straight tapped holes.

    The Williams Scope Mount Drill Fixture is an excellent addition to any gunsmith’s workbench.
    Last edited by M-Tecs; 05-01-2020 at 03:16 PM.
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  4. #24
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    Quote Originally Posted by M-Tecs View Post
    The Williams Scope Mount Fixture uses drill bushing.[/I]
    M-Tecs, your post reminds me I got lost in what I would do and forgot the info in the op's first post said he was using a drill fixture.
    Chill Wills

  5. #25
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    I once drilled and tapped two holes in my O3A3 Receiver to mount a Lyman #48 Receiver Sight. I chose to do this on a 4th of July and it was 100+ degrees here that day.

    I sweated blood for 4 hours to get 2 holes Drilled and Tapped in that receiver!

    It was case hardened about .020 inside and out. Not too hard to get the tap started but a real bear to get the threads all the way thru.

    I broke the same tap 4 separate times and had to dig the remnants out while sweating blood worrying that I had ruined a perfectly good un-fired 03A3 barreled action. I reground the end of that tap each time and it is now a Bottoming Tap Supreme.

    Every time I'd get the tap thru to the inside case it would snap the the end of the tap off like a toothpick, and more sweat would ensue.

    I finally got both holes all the way thru and threaded. Whew! I should also note: that I am normally a very lucky guy, hence the success in the end. There was also possible Divine Intervention involved despite the constant swearing in the shop.

    Please note: I was doing this on a milling machine where I could control the position and speed of the drill perfectly, and when tapping I had the Tap held in a Tap Handle with a Center in the spindle for perfect alignment above each hole. I was turning the tap with two fingers, with some Magic Tapping Spuge on the tap and in the hole. IE: This was the best possible scenario for doing this operation.

    Also this was not a cheesy tap it was an OSG 2 Flute Spiral Tip Machine Tap which are about the best and strongest there is. It was $25!

    I have another new one now but still keep the first one as a reminder that no matter how good you think you are you can still suck just enough on any given Sunday to make a mess.

    It should also be noted that I am (not Bragging ) very good at doing this type of thing. Still it was a 50/50 chance I'd screw it up. Also my holes were thru holes and I could punch the bits of the tap thru to the inside as opposed to a blind hole.

    I'll only do this type of work on cool days now!

    But my main point here is that Drilling and Tapping holes that are 1/8" deep and expecting to get any significant amount of threads in them is going to require some serious skills. You will be, under the best case, less than the diameter of the tap, so your threads will never reach the minimum depth to achieve full strength. If the holes were 3/16" deep you could do it, but 1/8" is not enough. #6 taps are @.136 on the OD, and your threads and the screws you put in them, will never get all the way to the bottom of the hole. Best case you might get .080-.090 worth of usable threads and probably more like .060. 6-48 threads are .021 pitch so maybe 3-4 threads..

    If I was you, I'd figure out a way to use the existing holes in the barrel and make an adapter plate to mount whatever on to it.

    Randy
    Last edited by W.R.Buchanan; 05-02-2020 at 05:34 PM.
    "It's not how well you do what you know how to do,,,It's how well you do what you DON'T know how to do!"
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  6. #26
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    When modifying the tap, keep the flutes as short as possible. the shorter the flutes and tap the stiffer and stronger it will be. If your tapping 1/8" deep leave the flutes 1/4" long

  7. #27
    Boolit Master Cap'n Morgan's Avatar
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    Randy, I remember only too well how frustrating it could be to drill & tap receivers!
    These days I use a CNC mill and it makes the procedure so easy it should be illegal. Placement and depth of the holes are held within .0005" by default. Holes and threads are milled using carbide tooling and can be adjusted to whatever diameter and depth you want. The threads are milled using helical interpolation and a single point tool. After a full circular no-pitch cut at the bottom, the thread is then cut from the bottom and up. This allows for the screw to bottom-out in the hole even if the screw thread has no taper at all.
    Cap'n Morgan

  8. #28
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    Never drilled and tapped a 03A3. Done lots of cased M98's and 1903's. The very early 03A3's weren't 8620 but most of them were. I have drilled and tapped 8260 after it was cased and that is the nastiest tapping that I have done.
    2nd Amendment of the U.S. Constitution. - "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

    "Before you argue with someone, ask yourself, is that person even mentally mature enough to grasp the concept of different perspectives? Because if not, there’s absolutely no point."
    – Amber Veal

    "The Highest form of ignorance is when your reject something you don't know anything about".
    - Wayne Dyer

  9. #29
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    As the OP, I want to thank everyone for their good advice & send a special "thank-you" to Assassin & Texas-by-God. Assassin provided the bottoming taps I needed & T-b-G sent me a base that was invaluable in my measurements & testing. I was very surprised that this thread raised 2 pages of interest. As for lessons learned, I should have posted my questions here BEFORE going off halfcocked & starting my project. I think being on stay-at-home restriction I'm starting to go a bit crazy! Never the less, here are some pictures & comments.

    Attachment 261788 Attachment 261789

    I bought a Weaver 92A aluminum base that would extend back over the top of the action. The rifle’s barrel is a straight 1” dia its entire length. I wrapped a layer of 320 sandpaper around the barrel & spent about an hour lapping the base down close to a good fit. I cleaned up everything then using epoxy, skim-coated the base & bedded it to the waxed barrel.

    Attachment 261790 Attachment 261791

    I mounted a Weaver K4 scope as a starter. I might switch it out temporarily with a higher-power scope for load development.

    Attachment 261792 Attachment 261793

    I like how it came out & I just hope it's not shooting 4 feet to the left or something...LOL! I'm anxious to find out....

    Now …can anyone recommend a good .256 Win Mag load?...
    Hard times create strong men, strong men create good times, good times create weak men, and weak men create hard times.

  10. #30
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    We need a "like" button for this whole thread, and especially the last post with photos. A job well done with LOTS of good advice and shared knowledge.

  11. #31
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    Fantastic job!

    Sent from my SAMSUNG-SM-G930A using Tapatalk

  12. #32
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    Glad I could help and you are more than welcome!

    Gun looks great! I hope it shoots as good as it looks.

    What bullets do you intend on running through it?

  13. #33
    Boolit Master pertnear's Avatar
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    Quote Originally Posted by ASSASSIN View Post
    Glad I could help and you are more than welcome!

    Gun looks great! I hope it shoots as good as it looks.

    What bullets do you intend on running through it?
    The rifle came with some 87 gr Sierras but I also have some 60 gr Hornadys & 70 gr Sierras I want to try. By the markings on the underside of the barrel, I think it is a 1 in 10" twist but I haven't verified that yet. I wouldn't mind shooting some cast boolits, especially if someone on the forum would care to make a recommendation. I made my first set of cases today from .357 Mag brass & didn't ruin even one case! But when I went to trim them I discovered the necks were too thick to go over the trimmer pilot. I'll need to do more research to see about that issue. Luckily the rifle came with a number of factory cases & RCBS dies so I can start reloading right away.
    Hard times create strong men, strong men create good times, good times create weak men, and weak men create hard times.

  14. #34
    Boolit Master pertnear's Avatar
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    Finally got to shoot & zero in the rifle today - despite a little rain. I set the seating depth .01" off the lands & loaded up 20 rounds with 87 gr Sierra & H4198. I used the book minimum of 14.0 gr. Once I got the scope zero-in I shot a final 3 shot group. I was very pleased as the 4X scope pertnear covered the whole aiming point at 100 yds!
    Attachment 262030
    Hard times create strong men, strong men create good times, good times create weak men, and weak men create hard times.

  15. #35
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    What does this little cutie weigh? I predict lots more fun with that one. Accurate guns are the funnest!

  16. #36
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    Good job! You should have a LOT of fun with that gun for sure!

  17. #37
    Boolit Master pertnear's Avatar
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    Quote Originally Posted by Texas by God View Post
    What does this little cutie weigh? I predict lots more fun with that one. Accurate guns are the funnest!
    You made me curious so I weighed & measured. Overall length 35" & has a 20" barrel. Weighs 8.0 lbs.
    Hard times create strong men, strong men create good times, good times create weak men, and weak men create hard times.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check