what is the safest depth to drill scope mounting screws

[corey012778]

i am working on an MN project. pretty much working on a tight budget and lack of gunsmiths willing or affordable. I am going to (well) bite the bullet and do it my self. I am going to practice on some scrap steel that should be close to the same thickness of the area I am wanting to do.

I am wanting to drill thru the mount I have, down to the rear sight dove tail, counter sink the holes on the base I have and mount that to the dovetail.

mount I am using


dovetail (work in progress, have a lot to do still, planning on cleaning up the outside and refreshing the blue or even rebluing the barrel)

[Char-Gar]

You can safely drill through the side wall of the action or the rear receiver ring for a side mount.

If you are using a mount where you have to drill on the receiver over the barrel that is another matter entirely. In that case, I would go no deeper than 5/32 of an inch. That is deep enough for screw to hold any mount against heavy recoil.

Please bear in mind that if a hole is drilled all the way through the front receiver ring, into the threads on the barrel, the barrel will be a booger to remove should you ever need to do that.

[pietro]

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Since your pic shows that you intend to drill into an area integral to the barrel, I would advise that the maximum depth your hole(s) can be drilled would be at a point half the barrel wall thickness from the edge of the bore.


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[LAGS]

Three full threads is a standard.
That gives you a hole drilled into the barrel about .100. to .120 max depth
I do not recomend drilling Half the wall thickness of the barrel especially just ahead of the chamber.
Your screws will probably be 6/48 or 8/40
Devide out the depth of each turn to see about how many threads you will have when the screw bottoms out.
6/48 is 48 turns equal 1" , 8/40 is 40 full turns equals 1"
Now remember, You do not want your screw to Totally Bottom out in the hole.
So a hole drilled .100 deep, will when tapped with a Bottom tap, get you three or even 4 full turns into the barrel
BTW, 6/62 and 8/32 screws are not a good choice for holding on sights.
The threads are too corse, and not as strong as a Finer thread 6/48.
But use a 6/32 to practice tapping on some scrap.
Replacement taps are cheaper and more availabe if you break one.
But do order yourself some Gunsmith size taps and drills from Midway or Brownell's if you havent already.

[corey012778]

I do have some on order can't remember what size

sounds like I should just go with the original plan, but may still practice. was hoping just to go into the dovetail section and not make it in to the barrel.

original plan was just make a spot for a set screw to set in.

[LAGS]

Now, Lets Measure your Barrel.
Measure the outside diameter of your barrel Just AHEAD of your rear sight.
On a MN it is about .760
Then subtract the bore diameter, say .312 and that gives you .448. then devide .448 by 2 and you have a barrel wall thickness of .224 at that point.
Drilling .100 deep is still under half of the wall thickness.
But Wait.
Now Measure the thickness of the barrel at the dovetail from top to bottom.
Take that number and subtract the Barrel Width at the same point on the barrel.
That will show you how much more metal you have on the Top of the barrel at the dovetail.
If it is a little bit more than the wall thickness, then you have a comfort zone when drilling from the top and could go deeper with your hole a little bit.
Now If you are using the screw to keep your scope base from sliding and not just using it to Totally hold it on to the barrel then you could use a 6/32 size and be fine
But max you would be able to get is 2 1/2 to 3 threads on a corser 6/32 screw in a .100 deep hole
Sheer strength would be ok, but Pull out wont be as much as if you used a finer thread.

[corey012778]

the metal stock I am going to use for practice is .12555, mn dovetail is .101 to the barrel. also looking at an exposed pin hole. took a close it is cut into the barrel a little. measured from the top to the bottom of the cut. .110

[LAGS]

If you are going to teach yourself to drill and tap, then I suggest you start off drilling a hole all the way thru something, and then Tap it all the way thru.
Do that several times, till you get the FEEL of what the tap cutting and binding feels like.
Then try tapping a Bottom hole. You will have a better idea of how a tap should feel, and get a sence of what it feels like when you get to the bottom of the hole and need to stop before the tap bottoms out and locks up.

I forgot that you have a 91/30 that has sight base retaining pin holes thru the barrel.
You are safe if you do not Drill any deeper than the bottom of the pin hole depth, measured from the top of the dovetail top flat area.
Good luck.
What Gunsmith Books did you order, ? I may also have the same ones.
You said you want to Reblue your rifle.
Have you thought about Slow Rust Bluing ? you can do that yourself at home for minimal expense and it is WAY more durable than any Cold Bluing Process. I do all my rifles in Slow Rust Blue on my kitchen stove or gass grill in the back yard.

[corey012778]

I got something called nu blue, going to try it on an muzzleloader trigger guard that I stripped the blue off of when I was taking care of a rust problem. not near any of that yet. wed. and thur. are busy days personally. plus I am taking it easy for a week or so.

one I can recall off the top of my head is gunsmithing made easy, the other two I had to pull up. gunsmithing rifles and do it your self gunsmith. just the start, going to order some more down the road. had a really old book don't know where it is.

[pietro]

I do not recomend drilling Half the wall thickness of the barrel especially just ahead of the chamber.

FWIW, neither did I - I only posited that depth as a maximum safe depth.


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[LAGS]

I do have the Gunsmithing Made Easy.
That is very informative, and has a chapter on building a Mauser Action and setting the Headspace on a Short chambered barrel.
I think that will come in handy for that Mauser you may want to Rebarrel someday.

@ Pietro.
Half way is the Max. safe distance you can drill into the barrel, As you correctly stated, But I use caution when instructing someone New, so they Fully understand How you measure a Barrel to determine what is the Actual wall thickness.
Like I suggested measuring the barrel Ahead of the rear sight.
Half that Wall thickness would be safe for the rear of the sight base diameter.
But if you measured the barrel at the rear of the sight base, and used that wall thickness for the Front screw, with a slight barrel Taper, you could be more than half the wall thickness.

Also, Half the wall thickness INCLUDES the Point of the Drill.
But the Tap, will not tap in that pointed area left by the drill point.
But when you measure the depth of the hole you drilled, it usually only reads the Shank or straight portion of the Hole, not the point area.
If they can work with a depth that is Less Than half the wall thickness, the better off they are

[pietro]

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Yep - If someone's going to err, it's best they err on the side of caution.


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[corey012778]

lags, I see way you suggest 6/32 screw. can find them just about every where.

think I have a game plan. do all my practice on the metal stock I have. I have to measure the width, but from eyeballing it. it is not far off. since I am planning on drill how thru the mount, I could place the mount on the metal stock and use it to set my drill press stop before trying it on my barrel.

for what I am doing I could just use a 10-24 or 10-32 screw. but good practice for any thing I want to do in the future

[LAGS]

You can practice with the #10 taps, then work down to the smaller tap sizes.
But a # 10 screw is a little big for mounting a scope base.
I am lucky, even though I have several screw sets for doing gunsmith work, I found that a local ACE Hardware store by my job stocks and sells some common screws in Gunsmith thread like the 6/48 , 3/56, and 8/40.
The selection isnt that good but it is a good place to go when I am running low on a certain size and am not ready to place an order with Brownell's or Midway, Nor have the time to wait for them to be shipped.

I looked thru my books, and do not have the other two that you mentioned.
But one reason I suggested looking in Goodwill or secod hand stores is,
Many of the books I find are Old and out of print. Many are from the '60's thru 80's
And many were written before we had all the Gunsmith Jigs and gadgets that you can buy now.
The Books show how it use to be done back in the day. But still is a good way to do things, and in a lot of cases , more economical for the guy just starting out. You learn to Hone your skills and not to depend on some fancy overpriced Jig or fixture.
Before I moved to Arizona, my wall looked like the Gunsmith's wall in the movie "Sniper" , a whole collection of gunsmithing books.
Come to think of it, I do also kinda look like him too.

[LAGS]

If you want to order more Books, I would suggest one called Gunsmithing Kinks 1 & 2
Lots of good tricks of the trade on all kinds of things.
You can learn about perticular Qwerks on a variety of guns.
I consult my books from time to time , just to come up with Ideas for other projects .

[andremajic]

16 oz of Andy's rust blue for only 12.00+shipping in swapping and selling section!

(Shameless plug!)

[LAGS]

Next go round, I may just give Andy's a try

[corey012778]

16 oz of Andy's rust blue for only 12.00+shipping in swapping and selling section!

(Shameless plug!)

shame shame

I have the 6/48 tap came in today. ordered a 6/32 tap just in case. do have a little experience in drill and tapping, did homemade ramrods. and drum for a muzzleloader but that has be a while ago.

had every measurement but not the base. well do now, thin area is .499 thickest .900.

[LAGS]

Is your 6/48 tap a, Taper Tap, Plug tap , or a Bottom Tap ?
If you intend to tap a hole in the top of the barrel , you will need a Bottom Tap
A .120 deep hole, the tapered tap point will hit bottom before it cuts one full thread.
Also, did you get the right size number drill for a 6/48 tap ?
Close is not good enough when doing fine threads or Gunsmith work.
BTW
Where are you getting those measurments on your barrel.
Is the .499 at the muzzle, or is that the wall thickness at the front of the old rear sight ?
And the ,900 ? where is that ?
Are you using a dial caliper to take measurements ?

[corey012778]

bottom tap, and yes I order the right drill bit.

dovetail scout scope mount. the set screw stinks. wanting a better mount set up. figure I can make it more rock solid.




duh, I did not measure the right areas