Drilling Holes in Firearms
The man comes into the gun shop and ask “How much to drill and tap a couple of holes in my rifle”. Ten dollars for the holes, fifteen for the threads and twenty for putting them in the right spot the first time! Then you need to smile and make sure you didn’t make him mad but you really told the truth. Like so many machine shop operations it is the setup that really eats up the time and requires the knowledge.
Normally we are talking scope mounts when drilling and tapping so it is like putting pistol sights on a 25 auto that will be sighted in at 200 yards. Huh? Look at the front and rear scope bases, now imagine they are a front and rear sight. Just like adjusting sights, the closer they are together the more the bullet impact is shifted. Most modern scope mounts will move the bullet one minute of angle when the base is moved .002″. A hair from your head will usually measure .005 so to center a scope on a rifle literally requires you to do better than splitting a hair!
Maybe the first job is to decide if the gun should be drilled and tapped at all. If the gun has true collector value it might be wise to leave it as is. Old Winchesters and such will lose value if modified in any way. Still some owner will insist on mounting a scope and after all it is their gun. Just do your best to let the owner know so they can make an informed decision.
Next is the actual layout and selection of mounts and bases. The cheapest is rarely the best and this applies to mounts as well. Try to select something that will look good and hold securely. Most of the time the owner preference will be the deciding factor but you might be able to guide him to a better choice if you are well informed. Most manufacturers supply charts to tell you which model base fits which firearm. Be aware that sometimes the are slight differences between models that can require a different mount. Sometimes you can modify bases for special applications or the correct things that went wrong from the past like a hole drilled off center.
Now you have the scope, bases, rings and barreled action. This would be a good time to center the reticule in the scope. Find a small cardboard box and notch the sides so the scope will rest in the notches with the adjustment knobs between the notches. Now slowly rotate the scope tube while looking through the scope at a distant object. When you have adjusted the scope so that the center of the reticule no longer moves as the body is rotated the reticule is centered. This is important because the scope performs best when you are looking through the center and it leaves plenty of adjustment for sighting in.
Now you can layout the bases on the receiver and make sure the bases look correct and the holes to be drilled will not be in bad locations like on the face of the bolt lug shoulder or through a working part attached to the top of the receiver. A lot of 22 rifles have bullet guides and such mounted in the top that can be avoided by moving the front or back a small amount. Just make sure the final location you select will work with the scope. Sometimes an extension ring can solve problems with base spacing. Using a pencil or tiny scribe marks go ahead and mark the locations of the bases for a reference.
If you have a Forster jig for drilling and tapping you can mount it on the drill press or milling machine and follow your directions for securing the barrel action and the jig to the machine. I can’t imagine doing this type work professionally without this jig. It make jobs that are difficult almost easy and saves so much time that it easily pays for itself in a few jobs. I have looked at other jigs through the years but have not found one that equals the Forster. Not saying there isn’t one but I haven’t seen it. It makes sure the holes are drilled in reference to the center of the barrel, makes sure the hole spacing is correct, makes sure the tap is held straight and serves as a depth stop for holes that can go all the way through. While holes that go through are preferred because broken taps can be removed a little better sometimes it isn’t possible and can be extremely dangerous to go through. A screw going into a chamber can turn a 6/48 base screw into a bullet that will go through a scope body and can injure a shooter. I have seen this more than once!
If you don’t have a jig of any type and are determined to drill and tap anyway I have a method for that too. Assemble the rings and bases to the scope you centered earlier. Now set the assembly on the receiver where you want it to go. A collimator or boresighter can should be used to make sure all is properly aligned. If you don’t have a bore sighting device you can look through the bore at a distant object and align the scope accordingly. Don’t forget if this is a long range target gun that you might want to allow for extreme drop. Many long range bases have 20 minutes of elevation built in the base. After making sure it will work in this location mix up a little five-minute epoxy. Now place a small dot of this epoxy between the receiver and base and don’t disturb until tomorrow. Yes, it said five minute but you want the maximum strength. Now remove the rings from the bases. Now select a drill the size of the screw heads and lightly turn this in the screw holes in the base. This will remove the extra epoxy and mark the center of the hole in the base on the receiver with a small start on the hole on the receiver. Now pop off the epoxied base with a plastic mallet and the receiver is ready to be drilled. The holes are started and it is fairly simple to keep the drill in the right location.
This jobs are high in nuisance value and eat up considerable time for the gunsmith. They also carry the threat of a lot of bad publicity if done poorly. Thankfully it has become a rare job for the gunsmith but it still shows up at the gunshop from time to time so a gunsmith will need to master getting holes in the right location. Remember it is less than splitting a hair! Do good work!
Timothy P. Whealton