M48 receiver hardness

C

cheapshot

New member
Has anyone here drilled and tapped an M48 receiver yet? I'm getting ready to drill mine, and I don't know whether to use a carbide drill or a steel drill.
Also, I'm not sure how deep to drill the front holes (blind holes). How much deeper than the screw protrusion should I drill to allow for end clearance of the bottoming tap. I'd hate to drill the hole and tap it only to find out I'm not deep enough. Do you gunsmithing pros use a bottoming reamer to square up the bottom of the hole?
The rear holes are obviously not going to be blind holes because with the lug channel there's only about .060"- .070" material thickness. I'm not too worried about these holes though, but the ones in front, where all the pressure happens, are the ones I'm worried about.
I had to order 6-48 taps from Brownells, so it'll be a few days before I can do it. BTW, have you ever noticed places like Lowes, Home Depot etc. have acres and acres of stuff, but for some reason, they never have the one thing you're looking for?
 
On those holes you start with a regular tap, then switch to a bottoming tap that has cutting threads almost all the way to the end. You can use a drill stop (clamps around the drill bit) or the drill stop on the press to make sure you don't go too deep. Some jigs also have stops.

I hope you are going to use a drill jig. Doing it by eyeball is not recommended for anyone, especially those without experience. (This is a polite way of saying, "You'll mess it up and the holes will look like a drunk's footprints"). You use the tap in the jig as well to keep from starting the threads crooked.

Mauser receivers are not hard and a good carbon steel drill will do the job, but I recommend carbide for everything.

Jim
 
Thanks Jim,
I've read many of your posts and know that you have a lot of knowledge about gunsmithing.
I will be using a carbide drill as you suggested. The're not cheap, but they're worth it.
What I'm wanting to do is intentionaly drill the holes shallow, then cut away that conical portion of the hole that the point of the drill bit leaves at the bottom of the hole, so that there is a minimal amount of clearance between the end of the screw and the bottom of the hole when tightened. It occurs to me that with a 6-48 thread pitch, I can get another two or three threads by squaring the bottom of the holes, and not have that conical empty space under the screw.
As far as using a jig for locating the holes, I really am a cheap shot. What I'm doing is a rather lengthy and complicated process that IMO will work as good if not better than using the jig. First, I centered the scope crosshairs via rotating and adjusting in V blocks. Then assembled the rings and bases into a unit, so that the bases are perfectly parallel. Then with the rifle in a vise, I centered the bore on a knot hole on a tree one hundred yards away. When I placed the scope/ring/base assembly on the rifle, I can see that the crosshairs aim exactly one inch above that knothole. It will only take 2 or 3 clicks (if any)for final scope adjustment when completed.
Now comes the tricky part. The holes in the bases measure .137" dia., but the drill for a 6-48 screw is only a #31 (.120"dia.). So I have to find the center of the holes first. My solution for that is to epoxy the assembly to the rifle, then spotting the holes with a #29 (.136" dia.) drill. Then remove the bases and then drill the spots with the #31 drill (in my drill press). Then I'll use the drill press to hold the tap and rotate it by hand. After I'm done with that, I'll permanently mount the bases with either JB Weld or red Loktite and take it to the range.
No, I'm not a gunsmith, but I am a machinist in the aerospace industry, and a lot of that translates to gunsmithing quite easily. I've done lots of things like this before; just not on guns specifically.
I certainly wouldn't recommend my method to anyone who has no machinist experience.
I do appreciate your knowledge and help Jim, and if you ever need help with bulding an F-15, gimme a call...:D
 
The standard mount screws are not long enough to need to worry about the extra thread you would gain by getting rid of the "empty space". Nor would it be significant or worth the trouble. If you want a stronger setup, go to a size 8-40 screw. There is actually no problem in the front mount if you drill through the receiver ring. The rear screw will usually come out in the bolt lug area, so it is not a blind hole. The front one can even go into the barrel a bit, but of course you can't let it go through into the chamber (and must remove it if removing the barrel).

Using the scope and sight base to locate the holes is a common technique with side mounts and works OK for an experienced person. You drill a front hole, then do a final adjustment before drilling a back hole. But for a top mount, I much prefer a jig. Maybe they do things differently in the aerospace industry, but I don't think the folks who make F-15s line up their work on a knothole.

Jim
 
Thanks again Jim,

I might use your 8-40 screw suggestion.
As far as using knot holes to align our work, we do in a manner of speaking. Except we use what we call datum points in conjunction with optical theodelite and laser transits to compile XYZ coordinates to locate points to within .0005".
Now, granted McDonnell Douglas (now Boeing) won't let me borrow their equipment to use at home, but I can mimic the same geometric principles at home in my shop.
Normally, we use jigs and fixtures when the prototypes are already built and the production line is running, and after all the math is done. It's when we're working on something new that we have to locate/layout things by hand, which I specialize in.
We don't use this particular brand but this is the same as what we use:
http://www.laseroptronix.se/ladar/todee.html
 
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