Drilling & Tapping a Barrel for a scope mount

[pertnear]

I have a single shot rifle with a barrel that I plan to drill & tap for a T/C style scope base. I've rented a Williams scope mount drill fixture & bought the appropriate drill bit & 6x48 tap for the screws I have. I plan to drill 1/8" deep holes & then I'll need a "bottoming" tap to thread these. Can I just grind my regular tap down to a flat tip to do this?

Any help or suggestions are most appreciated.

Thank you

[Tatume]

You're edging into risky territory here. If your rifle is worth much you would be better off to find a good gunsmith to do this for you.

[pertnear]

You're edging into risky territory here. If your rifle is worth much you would be better off to find a good gunsmith to do this for you.
I agree but all the local 'smiths are swamped, especially when comes to doing the little jobs. I don't want to ship the rifle off just for a drill & tap.

[Winger Ed.]

You can sort of make a bottom tap from a taper or plug tap,,,,,,,, however;

They aren't the same. The angle of the cutting edge is different.
You could get away with making a bottom tap like that to use in softer stuff, but a rifle barrel-- probably not.

Rather than trying to dig a broken tap out first, or wondering which oversize taps & screws will work after I striped the hole--
I'd just get the correct bottom tap to start with.

[Der Gebirgsjager]

Yes--that's how most bottoming taps are made. You'll want to grind it down to it's major diameter, not just a little off the end, so that it will cut threads as nearly to the bottom of the hole as possible. Use a cutting fluid when tapping, Tap Magic is a good one, but there are others. Turn the tap, regular/bottoming, clockwise and when you feel it tighten up back off a turn or two. It is tightening because chips being cut are jamming the threads, and if you force it you'll break the tap. A broken tap in a hole can be a problem to remove. An 1/8th inch hole is not very deep, but three complete revolution threads will get you 75% of maximum strength--considered enough in most cases. If you're using a drill press, then most of them have a device on the side for setting the depth. You might want to double check this for accuracy using a piece of scrap metal before doing the job on the barrel/receiver. A good way to set the device is to use a drill bit of the same diameter as the depth of the hole you wish to drill, in this case 1/8 and adjust the depth device to clamp that bit, then remove it and the device will then let the drill press handle turn just that distance and stop the downward progress of the bit drilling the hole. A size 31, if I remember correctly. Before starting the job you'll want to center punch the exact location where you want the hole to be. It also helps to choke up on the bit as far as possible in the drill press chuck to eliminate wobble.

[kenton]

In general, no grinding the end off of a tap does not make it a bottoming tap.
261358
note the bottoming tap still has some lead.

That said, I have in the past done exactly what you said and ground the end of a tap to finish a couple more threads but I probably wouldn't risk it on a gun barrel.

[Drm50]

I hope you are going to do this on a drill press. Not familiar with jig, how is top dead center located?

[LAGS]

I have taken old 6-48 and 8-40 broken taps , ground the broken tip off and used them to Bottom Out a hole that already was started to be tapped with a tapered tap.
But like was said by another.
Without the starting taper on a bottom tap , it can be difficult to start in a new untapped hole.
As far as depth.
Devide an inch by the number of threads per inch of your Tap.
Then multiply that dimension by the minimum number of threads you want or need to be secure.
And when drilling a barrel , don't over drill the depth of the hole.
You need to calculate the thickness of the wall of your barrel so you don't end up with a Thin Spot in your barrel or drill all the way into your bore.
There are lots of things that factor in when drilling and tapping a barrel or in some cases a receiver.

[pertnear]

I plan to use my drill press. I have a piece of cast-iron pipe the same diameter I hope to practice on. The Williams jig uses parallel jaws to center side-to-side. The rifle was drilled & tapped before for a long scope with 2-bases about a foot apart (Unertl style). I plan to use these existing holes to determine top-center line. The jig has a hardened guide for the drill bit & the tap. Part of my problem is that I can't just run out to the local hardware store & get a 6x48 bottoming tap (or any kind of 6x48 tap for that matter). The barrel is 1" diameter straight, no taper where the mount goes. The bore diameter is .257 & the drill area is past the chamber end. I picked 1/8" depth because that is what the existing screw holes are(?)
261360

[country gent]

If your only going 1/8" deep you may want to tap with a bottoming tap. A starting taps pint and relief is close to an 1/8" on a 6 48 starting tap it may bottom before getting close to full thread forms.

You can grind a starting tap to bottoming but it is not as simple as grinding the end flat, you need to grind some cutting relief on as well. Otherwise it will act more like an expungement tap pressing the threads in not cutting them.

Break chips often and clear often from hole. Break chips every 1/4 turn and clear hole every turn. As stated above a good cutting oil is a big help. once threads are started good pack holes with a wax consistency cutting fluid. as the tap enters it displaces the wax and pushes chips up out of hole.

[725]

pertnear
I've done stuff close to what you are purposing. It's doable. Take a long time with your set up. Practice a few times. Make your set up flawless. Have good lighting and no distractions. When it's game time, use a new tap and drill. Just being fresh makes it easier. Good luck.

[Texas by God]

I may get around to buying a true bottom tap someday. But so far my self modified regular taps have worked perfectly. All good advice above. I have a spring loaded center punch that I prefer to mark the spot with. Good luck with it, pics, pics.

[Jedman]

I do the same as country gent but if only drilling 1/8" deep I also grind a second drill bit with a nearly flat tip with relief like a endmill. You may get 1 or 2 more threads with a 48 thread tap.

Jedman

[skeettx]

You anywhere near Amarillo???
Mike

[pertnear]

You anywhere near Amarillo???
Mike

I live in the Highland Lakes area 50 mi from Austin. I'm pretty far away from Amarillo. :smile:

[LAGS]

But George Straight says "Amarillo By Morning"

[skeettx]

:)
Kinda far to drive for a few holes
Mike

[M-Tecs]

As others have stated yes you can grind a plug tap to a bottom. How effective that is depends on your ability and what type of tap you currently have. Spiral point or gun taps have better geometry for bottom grinding however they will not be a durable as a turn bottom tap. If you break the tap non bubba removal is not going to be cheap.

I would recommend spending the $10 to $15 get a true bottom tap.

https://www.amazon.com/6-48-bottom-tap/s?k=6-48+bottom+tap

[Bent Ramrod]

I grind the end off an extra or broken tap for bottoming those blind holes. I want every partial turn of thread I can get.

For the tapping, I start with a plug tap, continue with a normal bottoming tap and then use the ground flat tap for the last partial turn.

I also grind an extra tap drill flat on the point, and worry the bottom edges with a diamond file (trying to sharpen it like a milling cutter bottom) to clean out the conical bottom of the blind hole that was left by the regular drill.

I only get about four turns in those holes anyway. I want a straight cylindrical hole with threads all the way to the bottom, filled with the screw, so I also grind as much of the taper off the ends of the screws as possible, while still allowing them to start. The remaining space is filled with Loc-Tite.

Having a scope block come slightly loose in the middle of a match is a disheartening experience, often not easy to diagnose just then.

[243winxb]

:popcorn:

[Chill Wills]

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.

[ASSASSIN]

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.

[M-Tecs]

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/tnpla/79782710?cid=ppc-bing-New%20-%20Holemaking%20-%20PLA_UZM8NnNE__be_73667328438919_c_&mkwid=UZM8NnNE|dc&pcrid=73667328438919&product_id=79782710&msclkid=ab8b453b0cd712102fdb1b9766c62056&utm_source=bing&utm_medium=cpc&utm_campaign=New%20-%20Holemaking%20-%20PLA&utm_term=4577266906866425&utm_content=Center%20Drills%20(Combination%20Drill %20%26%20Countersinks)%20%26%20Spotting%20Drills%2 0Ad%20Group%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/williams-scope-mount-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.

[Chill Wills]

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.

[W.R.Buchanan]

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 :mrgreen: ) 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

[country gent]

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

[Cap'n Morgan]

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.

[M-Tecs]

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.

[pertnear]

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.

261788 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.

261790 261791

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

261792 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?...[smilie=w:

[KenH]

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.

[Texas by God]

Fantastic job!

Sent from my SAMSUNG-SM-G930A using Tapatalk

[ASSASSIN]

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?

[pertnear]

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.

[pertnear]

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!
262030

[Texas by God]

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

[ASSASSIN]

Good job! You should have a LOT of fun with that gun for sure!

[pertnear]

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.