Knowledge question for firearms thread

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Black Jack

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I have a Ruger MKII that I want to install a scope base. Found the info to proceed and it says to use a 6-48 thread. Why is it that firearms always use an uncommon thread pitch? What is wrong with 6-32 or 6-40? Same goes for the muzzle brake. 1/2x28 or 5/8x20. So now I need to find a tap 6-48 which you can't find just anywhere. So since I have to order one for that job, what other size is used commonly by gunsmiths that are not standard size. Also, for you gunsmiths that already did install a scope base like this, where do you indicate yourself to make sure you are square with the top of the action? I was thinking of using the square section in the back where you pull the bolt or using the hole that holds the frame and the action together. Is there a better way? Tks

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The finer threads allow you to put more tension on the screws which makes them less likely to back off during recoil etc. In thinner metal, it allows more threads to be in contact.

Fine threads aren't stronger IMHO and strip easier.

If you decide to use coarser threads that you mention, likely they will work just fine. I have done this with firearms that I keep for myself. It's often a good way to fix scope ring threads.


I also use a milling machine to do mine but if you don't have one, most don't, a drill press will work fine. You can even do it with a hand held drill if you're very careful to do the job properly.

The biggest issue with drilling an tapping such small holes is that the drill bits themselves will bend under the least bit of pressure, then wander off the punch marks.

There has to be extreme care used when applying the punch to the surface to be tapped. If the surface is hardened and the punch point is to large or hasn't been ground properly it will wander off the mark when impacted.

I don't use a punch any more. I screwed up one receiver and that was enough. Now, I use a centering drill bit, which has a very stiff and oversize stem with a small bit on its cutting tip.

I also grind the cutting face so that it is very pointy and as such is self centering and doesn't require a punch point.

A good square set of holes that are aligned with the center of the bore at the top dead center of the receiver or barrel is extremely important to proper scope mounting.

Most people drilling and tapping holes don't take the time and don't know how to properly grind/sharpen their implements. Very few know how to or even realize that the tap itself needs to be dressed after each hole. It only takes a few minutes with a proper stone to do this and the results do show.
 
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It's going to be done with my bridgeport. I am a machinist so I understand the fact about more thread per inch so more contact so stronger. But between a 6-40 and a 6-48 I have a hard time to believe that

the 48 is a lot stronger. Maybe it's to stop regular guys doing work with regular tools you can find easier but if you are equipped to turn a barrel for a muzzle brake then the fact that you cut it for 1/2-20 fine

thread or 1/2-28 will not change anything. It's not easier or harder. It's just gun world threads I guess. Well, it is what it is. :) Bearhunter, where did you get the stones to dress your taps. Did you buy them

or shaped them yourself. At work it could be useful because we throw them in the garbage when they don't cut anymore. The companies that sharpen the big drill bits and the endmill can also do the taps but

for 1/2 and lower it's almost cheaper to buy new ones than to have them resharpened. Tks
 
The 6-48 is pretty much the industry standard for mounting bases. All bases come with 6-48 screws ( unless 8-40).
 
Black Jack said:
I have a Ruger MKII that I want to install a scope base. Found the info to proceed and it says to use a 6-48 thread. Why is it that firearms always use an uncommon thread pitch?
Click to expand...

6x48 is an extremely common thread on firearms dating back to when factories first started tapping for scope mounts. I don't believe it is used anywhere else. Why did it become common and unique for North American scope mounting? Probably because an engineer felt 48 threads was easier to cut into steel over 32 or 40 threads.
 
I've never had occasion to drill and tap a Ruger pistol.
But almost all the holes I've drilled and tapped in firearms for mounting scope bases have been done using a Forster jig.
The piece can be trued and securely clamped in place. The tap drill and tap are guided top dead center by hardened bushings. Hole spacing can be precisely controlled.
If I were going to d&t a round receiver without the jig, I would certainly start the holes using a center drill, as bearhunter has suggested.
 
There is engineering behind the finer pitch. At a certain vibration frequency the coarser pitch thread will start to loosen before the finer pitch thread. You can read the research paper if you do a search with Google but it has lots of math in it.
 
For a given screw size the finer thread screw has a larger minor diameter and therefore is stronger. For instance 6-32 screws are very easy to twist off because of the smaller minor diameter. Also the finer pitch allows an extra thread or two in shallow blind holes.
 
tiriaq said:
I've never had occasion to drill and tap a Ruger pistol.
But almost all the holes I've drilled and tapped in firearms for mounting scope bases have been done using a Forster jig.
The piece can be trued and securely clamped in place. The tap drill and tap are guided top dead center by hardened bushings. Hole spacing can be precisely controlled.
If I were going to d&t a round receiver without the jig, I would certainly start the holes using a center drill, as bearhunter has suggested.
Click to expand...
This is how I did mine.
 
I don't have one of those jigs but I will explain how I was planning to do this job. First I have a Bridgeport milling with a DRO. So between the flat faces on the side of the receiver

and the hole for the pin to line up the grip and action. I will trust the pin hole more than the flat sides. I will machine a pin the exact size of the hole and then will use a collet with

this pin and insert it in the action. All this while holding it in my vise with parallels underneath. This way I will be perfectly in line with the original hole in the action. Set the DRO

to zero and we have a line up for the 3 holes. Switch to a drill chuck and proceed. I always use a center drill when working on a round to make sure everything stays where it's

suppose and doesn't wander around. Dang.... just looked on the price of those Forster jigs. 500 us$. I think I will stick with my plan for this one. :)
 
Black Jack said:
It's going to be done with my bridgeport. I am a machinist so I understand the fact about more thread per inch so more contact so stronger. But between a 6-40 and a 6-48 I have a hard time to believe that

the 48 is a lot stronger. Maybe it's to stop regular guys doing work with regular tools you can find easier but if you are equipped to turn a barrel for a muzzle brake then the fact that you cut it for 1/2-20 fine

thread or 1/2-28 will not change anything. It's not easier or harder. It's just gun world threads I guess. Well, it is what it is. :) Bearhunter, where did you get the stones to dress your taps. Did you buy them

or shaped them yourself. At work it could be useful because we throw them in the garbage when they don't cut anymore. The companies that sharpen the big drill bits and the endmill can also do the taps but

for 1/2 and lower it's almost cheaper to buy new ones than to have them resharpened. Tks
Click to expand...

It's way to much work to try to sharpen a tap. A good brand of tap will last a while, especially for a home gunsmith.
They are fairly cheap
 
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