I am guessing that the proper way to calculate where to drill some holes in a set of scope bases would be to first screw the proper screws into the receiver and measure their outside distance. Next, measure each screw and divide that measurement in half. Next add those two numbers together and subtract the result from the outside distance measurement to come up with distance between centers where the holes should go.

Did I leave out anything I am going to regret?

It doesn't need to be so complicated. Measure the diameter of the screw. Insert the screws. Measure the distance between screws inside to inside and then add the diameter of the screw. Or, if you measure outside to outside, then subtract the diameter. That's assuming the screws aren't wobbling in the threaded holes and you are able to get a solid reading. If not, and you have access to one, set the receiver in a milling machine and use a pin gauge to measure the distance.

http://www.youtube.com/watch?v=7-_eglLgUXg

I have done it on the mill, and I have done it on the drill press with a drill and tap fixture.

With the fixture I don't have to think.
It fits on many mil surplus bolt actions.

I still have to do Sav99 actions on the mill, and I have to think then.

I just watched the Potterfield video clip on drilling and tapping, I got to tell you, drilling a scope base hole with a power drill would scare the hell out of me.. I thinks it worth taking your receiver to a machinist or gunsmith to do this kind of work.

quote:

Originally posted by Mark_Stratton:
I just watched the Potterfield video clip on drilling and tapping, I got to tell you, drilling a scope base hole with a power drill would scare the hell out of me.. I thinks it worth taking your receiver to a machinist or gunsmith to do this kind of work.

I'm sure if someone could find a way to chuck a gun barrel in one and had a good way to hold the reamer without losing a thumb they would have a video on DIY chambering too. There isn't a better floating reamer holder than ones arm joints and sockets.

quote:

Originally posted by Mark_Stratton:
I just watched the Potterfield video clip on drilling and tapping, I got to tell you, drilling a scope base hole with a power drill would scare the hell out of me.. I thinks it worth taking your receiver to a machinist or gunsmith to do this kind of work.

The receiver already has holes in it. I am talking about scope base holes matching up exactly with the receiver holes.

No need for math, just measure from any part of one to the same part of the other. Front, back, screw head it doesn't matter, its all the same. A dial caliper works fine. (Assuming the screws are the same size)

quote:

Originally posted by 22WRF:


The receiver already has holes in it. I am talking about scope base holes matching up exactly with the receiver holes.

Run a screw in each hole, measure the outside distance over the screw's threads, subtract the diameter of the screw. That will yeild the C'line distance between the screws. If the holes were properly laid out down the centerline of the reciever that's all you need, but if the holes are crooked or off the c'line other measurments for alignment will most likely need to be done on a gage plate or in a milling machine using it as a measuring device with a center finder.

For quick and easy you can use a set of transfer plugs, these screw into the holes with fine sharp points just above the surface of the part with the threaded holes. Lay the mating part over the desired location and tap the part down to transfer the location of the threaded holes. I use these on occation makeing replacement parts for fixtures and gages, but have not applied this techinque to scope mounts, I doubt that perfect alignment could be held.

You'll get better results with direct measurement on a gage plate.

What we know from the gunsmithing book MacFarland about Mauser scope mounts:
Weaver #46 two piece mount for the front has .860" hole spacing.
Weaver #45 two piece mount for the rear has .504" hole spacing.

My testing:
Someone on AR just sent me a Mauser action with 3 scope mount holes.
I put it in the fixture with a #31 drill the one rear holes as a locator pin. I tightened up the fixture and drilled and tapped the 4th hole [other rear].
That rifle is now drilling holes in paper with a #46 and #45 mounts shimmed, epoxied, and Loctite on the threads.

My Goggling and measurements for this post:
The one piece Farrell Mauser mounts in 2000 had 3 holes. In 2009, they now manufacture with 4 holes.
The one piece EGW Mauser mounts have 4 holes.
http://www.kenfarrell.com/FN-MAU-1-0.html
http://swfa.com/EGW-Picatinny-...pe-Mounts-C1681.aspx

If I put a .150" pin gauge in the furthest forward hole in an Farrell and .151" pin gauge in the rear most hole, stand the mount up on the granite surface plate against a ground "L" bracket, and measure with the height gauge, the holes are 5.130" on centers.

If I put a .143" pin gauge in the furthest forward hole in an EGW and .144" pin gauge in the rear most hole, stand the mount up on the granite surface plate against a ground "L" bracket, and measure with the height gauge, the holes are 5.182" on centers.

If I put a .315" pin gauge in the furthest forward hole in an Wheeler Engineering drill tap fixture [clone of Square B fixture no longer made] plate #1 without drill or tap bushings installed and .316" pin gauge in the rear most hole, stand the mount up on the granite surface plate against a ground "L" bracket, and measure with the height gauge, the holes are 5.633" on centers.


What does it all mean?
For Mausers, the distance between the two front holes is fixed at .860"
For Mausers, the distance between the two rear holes is fixed at .504"
For Mausers, the distance between the front most hole and the rear most hole is cook's choice 5.130" ~ 5.633".

What does all THAT mean?
You drill and tap two holes in front where you want them, then reposition the fixture so the rear 2 holes will be where you want.

ONCE AGAIN, I am not trying to drill and tap a receiver. rather, I am trying to make a set of custom scope mounts and want the holes to line up perfectly. No more and no less.

Why don't you get a center distance caliper or a center distance attachment for a set of calipers. That way you get the precise screw hole distance of your receiver.

Getting the holes in you bases is the same process theory as drilling and tapping a receiver. Just apply what the other posters have conveyed to you about that process to your custom bases.

You may also want to check the ID of some commercial mount holes. You don't want to trap yourself into uneccessary tolerances for something like scope bases.

quote:

Originally posted by 22WRF:
ONCE AGAIN, I am not trying to drill and tap a receiver. rather, I am trying to make a set of custom scope mounts and want the holes to line up perfectly. No more and no less.

I give up.

But I will tell you a joke.
In production tooling in Boeing in 1990, if more than two guys were talking, it was because the holes didn't line up.

In 2005 at General Dynamics, there was a technician that every test fixture he built for me, the holes lined up. I gave him a surplus Mauser still sealed in the Century box.

quote:

I give up.

I don't blame you.

But I think you assume that the action I am working on was factory drilled and tapped.

first....
are you making a 1 piece or a 2 piece base?. it matters as to "how many" center measurements you'll need..
for 2-piece bases, i would....
find some screws that have some shank on then and run them in one set of holes till they snug up on the shanks. measure out side to ouside of the screws, subtract one shank diameter. the remainder will be the centers of that set of base screws. repeat the process for the other set.
to find centers of one screw in front ring and bridge (in the case of a 1-piece base),.... run screws in front most and rear most screwhole, measure out side to outside and subtract 1 shank diameter.

It took me many minutes to make measurements with pin gauges and height gauge on a surface plate.

I verified them in a couple seconds by measuring a hole, and zero the digital calipers on that hole size.

Then an outside to outside measurement with the calipers reads center to center.

Still no calculation and 10 seconds instead of 10 minutes.

I have milled out Weaver rails from from scratch in Aluminum.
That is allot of work.
Much easier to take some AR15 riser and hack it up.

WestPac

quote:

Measure the distance between screws inside to inside and then add the diameter of the screw. Or, if you measure outside to outside, then subtract the diameter.

Malms righr on or you can measure inside to outside then add or subtract nothing.

quote:

Originally posted by 22WRF:
ONCE AGAIN, I am not trying to drill and tap a receiver. rather, I am trying to make a set of custom scope mounts and want the holes to line up perfectly. No more and no less.

And no less than three of us have told you how to do it.

I'm with tnekkcc, I give up.