Gunsmiths Do Make Mistakes Sometimes

There have been many articles written in both trade and hobby gunsmith literature about drilling blind holes, tapping, extracting broken taps and the like. All readers who aspire to any skill at all with routine tasks in the gun shop are encouraged to read and soak up all there is on this important subject as it is very important stuff. This article begins with the correct way to perform this work in the shop.

Following that is a story of what actually happens when things went awry for a well-intentioned gunsmith being just a little too casual in his work methods and managed to mess up such a hole. For those not familiar with this work, there are many ways to accomplish a threaded blind hole in a barrel. This is this is how one gunsmith approaches it with a few variations included for those of you who like variety in gunsmithing.

The work properly includes first making sure the barreled action has witness marks where the barrel joins the action so the front sights can be more easily maintained at their correct twelve o’clock position if the barrel is ever removed and replaced. This doesn’t have to be done until the time of removal when the barrel and action are stamped together. The mark is usually placed where it will be covered by the stock where it won’t show, typically at six o’clock where the barrel joins the action.

Although the above is good practice, you may wish to ask the customer before you actually pound a permanent mark into his barreled action as some might consider it defacing their gun. After marking, the unit is ready for the real work. Now to the drilling and threading. There are the two basic procedures. The first involves using a Forster Universal Sight Mounting Fixture and the second uses a regular drill press vise.

If using the Forster fixture, the barreled action is carefully leveled fore and aft in the jig and clamped into place using the provided clamps in preparation for drilling and tapping. Side to side leveling is automatic when clamping a barreled action in place. Next, the hole’s location fore and aft is determined and the jig properly adjusted so a hole guide is located where desired. Now the hole guide part locking screw is tightened.

One does need to use the drill press depth stop when using the jig, however, the only way to mess up using the jig is to use a too large drill guide bushing, resulting in drilling the hole in the wrong place, or neglect to adjust the drill press depth stop correctly and drill too far. Using the drill press depth stop properly adjusted and a suitable drill tap bushing in the jig yields a hole is drilled to the correct depth perfectly centered at twelve o’clock in a single operation. Easy, wasn’t it?

All this was done more or less automatically by the jig. The jig is recommended because of the ease of use but a drill press vise using the following technique will do just a good a job. There are flat locations on the bottom of rifle actions such as the Enfield, Mauser and Springfield which make leveling easy. Using a good quality machinist’s level such as a Starrett, Brown & Sharpe or Lufkin, level the barreled action both side to side and fore and aft and then securely clamp it in the drill press vise.

Next, place the ramp on the barrel in its desired location fore and aft and level side to side. In the below example, Mr. Customer was very specific about how far back from the barrel’s crown the ramp was to be installed. Exact front sight ramp location fore and aft was specified as critical because of the choice of stock.

This arm was to be full stocked, fitted with a steel fore end cap and the front sight had to be in a location which would give the appearance of a classically-stocked Mannlicher-Schoenauer sporting carbine. The customer is always right, remember? The ramp is carefully leveled. For this task I prefer a Starrett No. 130 level (starrett.com). The ramp is then securely clamped in place while still level, checking the mounting hole isn’t blocked by the clamps.

I like to scribe the outline of the future hole in the barrel with a sharp scriber through the mounting screw hole in the ramp while it’s clamped in place, a technique I learned from Clyde Baker in his book Modern Gunsmithing. We’ll need a good drill press with a proper functioning depth stop. James Virgil Howe in The Modern Gunsmith describes his recommended approach to sight installation in the chapter, "Fitting of Sighting Equipment." To paraphrase, the

The barrel will be blued later. Hardened steel and easily sharpened. barrel or barreled action is leveled and securely held on the drill press table and the screw-on ramp base is securely clamped in place to guide the drill. He states that it is best to use a body drill of such size that it will just pass through the sight base hole and "spot" or just dimple the barrel in the correct hole location. Note that this spotting drill is much too large for anything more than just starting the hole.

Because the drill just fits the hole it can’t wander, so the hole has to start in the correct location. The dimple serves as a substitute for a center punch mark to prevent wandering when drilling into a curved surface. Mr. Howe discourages removing the ramp and instead instructs to leave it in place in order to use it as a guide for the remaining work.

He does state very clearly and correctly to next remove the oversize "spotting" drill and change to a correct size tap drill, a # 31 gauge drill for 6-48 threads typically. This drill is smaller than the hole but won’t wander if the dimple was deep enough to be of the full tap drill diameter. W.

F Vickery, regarded as another old master of the art, notes in his classic work Advanced Gunsmithing, "A drill that just fits the screw holes through the sight base is used to spot these holes, just drilling [to] the full depth of the drill point…" This depth can be limited using the drill press depth stop or simply estimated by eye. Some practice on scrap may be in order here if needed. The idea is to start the hole in the exact location desired and drill it true. Most front sight ramps these days come with a Mr.

Howe was not describing a front sight ramp installation but instead the installation of a Lyman 48 receiver sight on a US Model 1903 receiver bridge, his installation being on the right side of the receiver bridge. Receiver peep sights reigned supreme in his day, which was before scopes came into their own, however, the same rules of sight installation apply. The next part is very important. Before actually drilling the hole with the tap drill, set the drill press depth stop.

Too deep is all the way into the bore itself. Double check this. After assuring the stop is correctly set to not drill too deep, go ahead and drill the hole to full depth. Next, we’ll

130. switch to a bottoming drill, still of the tap drill size, which cuts a flat bottom hole instead of the usual 135 degree cone pointed drill which would have drilled a hole closer to the rifling at the center than at the perimeter. Drill with the bottoming drill just enough to get a flat bottom hole. If properly constructed, the bottoming drill won’t drill beyond this, unless it is sharpened like an end mill.

A simple bottoming drill can be made by grinding a regular twist drill in the correct size perfectly flat on the end. The barrel now has a flat bottomed hole ready for threading. You didn’t break through did you? Incidentally, there were no Forster Sight Mounting Fixtures in production when Mr. Baker, Howe, and Vickery were authoring their fine works on gunsmithing, but regardless of method, there is nowa blind hole in the barrel of correct size, depth, and location.

Before moving on, let’s look at a somewhat different method. Call it the center punch method if you like, this doesn’t require a Forster jig either. The above method is followed to the point when the circle is scribed on the barrel. Now, remove the ramp base before drilling. Recall previously the very carefully-determined exact hole location on the barrel was scribed through the mounting hole in the ramp. Use layout fluid or a magic marker first to make it easier to see the scribed circle for what comes next.

Very carefully centerpunch the holes location in the exact center of the scribed circle. The use of magnifying eyewear is recommended. If it is perfectly centered in the scribed circle, start drilling the hole, using a bit smaller than the tap drill size. For example, a #45 gauge size for a #6 thread is good. Of course, all drill press depth stop setting cautions apply. Stop after drilling to the depth of the cone and carefully inspect the work.

If the hole appears perfectly centered by eye when observed with magnification as before, proceed to drill to full blind hole depth. If the center punched hole isn’t perfect, "walk" the center punch by angling The hole sizes are for common tap body diameters.

it slightly to the barrel’s surface and deepen the punch mark by successive light blows until it is perfectly centered again. Working slowly and carefully is called for here. Sort of a trial and error and correct error procedure. At an average gunsmith’s charge of $25 to $50 a hole for drilling and tapping, depending on complexity, there is really no reason to hurry. The best cure for a heavy hand with a hammer is a smaller hammer. Use of really small ones are in order.

A four ounce ball peen or cross peen hammer is a good choice. Drill again but stop as soon as the drill has found its way. Repeat the inspecting procedure as above and if the hole is advancing and is centered, only then proceed to full finished depth using the smaller drill. If for some reason after all the above method is used but the hole is still not centered the following procedure comes into play.

Use a Dremel Moto-Tool hand held with a fine steel cutting point and grind the #45 drill hole slightly oversize, bringing it back to being perfectly centered by eye in the scribed circle. Then drill to tap drill size and clean the hole in preparation to cut the threads. It’s nowa perfectly centered correct tap drill size hole in the scribed circle and it’s drilled to the correct depth.

It is generally considered good practice among gunsmiths to allow one thread diameter minimum between the bottom of the hole and the lands. A #6 thread has a major diameter of 0.137 inch and a minor diameter slightly less. Whether major or minor diameter is intended can be debated, but not here. Attaining this minimum can be difficult with a thin barrel at the muzzle. Next, the hole is to be threaded.

This is done using a series of taps, taper, plug, and bottoming if it is a deep enough hole, just plug and bottoming only if it is a less deep hole and maybe just the bottoming tap only if it is very shallow hole such as our blind hole. In the incident described below, the tapping process

This was done before repair to "see" the dimple in reverse and after repair to confirm it was removed. Gunsmiths are into recycling, too. is where the problem arose, not the drilling. More about that later. If we were careful while drilling a very good job was the result. Now it’s back to using the jig or alternately using a substitute method for threading. First with the jig.

After drilling and cleaning the hole with compressed air, pipe cleaners or other suitable means, the drill bushing in the jig is exchanged for a larger tap major diameter size bushing which is just a little larger than the major diameter of the tap. With this bushing in place and a clean, well-lubricated, sharp tap in a Tee handle tap wrench, begin to tap the hole. Advance the tap a half turn, back up a quarter turn and repeat until the hole is threaded as far as the taper or plug tap can advance.

Don’t forget to remove and clean the tap with compressed air and/or a tooth brush as needed so the tap doesn’t become clogged with metal chips. When the tap has bottomed and can advance no further, back it all the way out, clean the hole until completely clean. Exchange for next shape tap in the progression, plug or bottoming, and repeat the procedure as necessary. When the hole is threaded all the way to the bottom the job of threading is done. The hard part is done. Hurrah!

Now, if not using a jig to guide the tap the threading is done using a handy tool called a tapping guide. It is simply an inexpensive V-block with suitable size holes for the tap shank sizes drilled thru the apex near the ends. It was convenient to locate the holes near the ends of the block. Two were made from blocks with two different size holes in each and have served me well for many years. One End of story.

One of the clamps is a home made replacement. of these blocks, when placed on around barrel serves to guide the tap so it is at a right angle to the center of the barrel. Starrett and other makers, including the "Made In China" brand, make inexpensive Vblocks and these are entirely adequate for making into tapping guides. Blocks that are not hardened and tempered are easier to drill.

Using this method, a suitable new, sharp, and clean, tap held in its holder is lubed with good cutting oil and inserted into the tapping block hole that fits it most closely. The block can be either hand held onto or clamped in place onto to the barrel. While hand held works very well, if

using the clamp, before tightening it, line up the tap’s point with the hole just drilled so the tap can be hand turned and backed up easily. Proceed to cut the threads in the usual manner. Note the tapping guide if held in place does just that. It guides the tap. That is it keeps the tap at a right angle to the barrel and aligns the tap with the center of the barrel. This is what is needed. So much for the tapping guide method. Don’t want to use a tapping guide? Don’t have one and haven’t made one?

Another way to insure the tap goes straight into the hole is to retain the barrel in the vise after drilling. Don’t unclamp anything. Exchange the drill in the drill press chuck for the first tap to be used and, using the drill press feed, gently advance the quill and turn the tap by hand, backing up as needed. This method also will start the tap into the hole perfectly straight. After a turn or two of this the tap is firmly enough held in alignment so it will continue to thread straight.

The chuck is then loosened, the quill withdrawn, the assembly taken out with the tap still in place. The tap can then be secured in a tap holder and the threading completed by hand. If no intentional side play is put on the tap while hand turning it will thread true to how it was started in the drill press. Complete the threading process and clean the hole. There is nowa properly threaded hole. So from this somewhat lengthy narrative, choose the tools and techniques that work best.

The use of special hardened drills, tapping lubricants, procedures for spot annealing, drilling near bolt lug recesses and the like are interesting, related subjects but best left for another time. What Went Wrong The second part of this discourse concerns an error made in threading a blind hole at the muzzle of a thin barrel. It was part of a routine installation of a currently-available front sight ramp.

This happened after many years of drilling and tapping barrels and receivers for all variety of sights and scope mounts, which just goes to show one can’t be too careful in this business. The barrel was quite thin at the muzzle necessitating very careful work in drilling and tapping a shallow blind hole for a screw-on ramp front sight so as not to break through into the rifling or cause any other damage.

Any damage to the all-important last little bit of rifling near the muzzle will affect the ability of the barrel to guide that little bullet straight and true to its intended target. At the risk of too much repetition, careful use of a good functioning depth stop on the drill press is called for so as not to drill too far and cause serious damage. Other cautions apply as will be seen. Everything started out fine.

Awhile back a customer brought into the shop four small ring, 1895 Chilleno-marked Mauser barreled actions in 7x57mm intended to be handed down to various family members in the almost forgotten family tradition of handing down cherished items. Each rifle was to be individually made up to an individual family member’s taste. A nice gesture I wish I would have thought of myself. The barrels were to be drilled and tapped for identical screw-on ramp front sight installation.

Having done this kind of work many times no problems were anticipated. An acceptable price was agreed upon and the job was taken. These were thin barrels at the muzzle and screws allowing a minimum of four threads to hold the ramps were selected. It is considered good practice to get a full thread diameter of thread length because that is the generally accepted amount needed for full holding power. A thread diameter length of threads for a #6 thread is 0.137", which comes out to 6.5 threads.

This barrel wall thickness allowed for only four threads when leaving enough metal at the bottom of the hole for strength. A similar rule of thumb for the adequate amount of metal under a blind hole near the muzzle of a high powered rifle is not so easy to come up with but the old one thread diameter of metal between a blind hole and the bore is a good starting point.

This also was not possible due to the thin barrel wall as it wouldn’t allow a full #6 thread diameter of threads between the bottom of the hole and the rifling while getting enough threads for adequate hold, so only a thin layer of barrel thickness would be left under the blind hole to the bore. Still, I felt confident the thin layer would suffice as my customer was only going to use standard pressure loads.

As an aside and to support this, elsewhere in Howe’s book he states pressure in the barrel at the muzzle is only about a tenth that at the chamber, based on testing by him while he was heading up small arms development for the US military and verified by others Frankford Arsenal. So, a thin web of metal is enough at the muzzle. The alternative ramp attachment method is to soft solder—not usually recommended—or hard solder, which has the risk of descaling the bore due to dull red heat needed.

Three of the four units were completed with very satisfactory results. The installed sight ramps checked out straight and level on each barrel with nothing showing in the bores. When it came time to do the fourth sight installation the same procedure as above was used … so I thought. A quick check of hole depth before tapping the last barrel revealed all four barrels were drilled to the same depth. Nothing showed in the bore.

I say this because I checked all the holes as drilled with a Brown and Sharpe depth gauge with the cone point accessory rod so the four hole’s depths were uniform as would be expected on identical barrels using identical clamping, same drill and retaining the drill press depth stop setting.

The holes were all drilled with a regular pointed twist drill. A bottoming drill, which was considered good practice by Mr. Howe, was not used to finish the holes. So we have a hole with a 135 degree cone point on its bottom as with the others not breaking through in a light contour barrel. There is adequate barrel thickness here for about 1/8" total of hole and thread length to the point of the cone point drilled hole, no more. Now to tapping, which you are all experts on from reading the above.

A bottoming tap was used which hadn’t had its blunt cone point ground off to a flat end. This tap was used to both cut and bottom out the threads on the hole in the last barrel. Apparently, it was turned enough at the end against resistance, as felt at the hole’s bottom, to very slightly dimple the bore on the last barrel. This is not a good thing! Not noticing the dimple at this time, all the barreled actions were returned to Mr.

Customer who soon brought back the one with the dimple in the bore with a repair request because the dimple was "noticeable." I ruminated for awhile and really wasn’t quite sure how to proceed. My mentor is now long gone and couldn’t couldn’t ask him, but time can be a friend in these situations. The customer indicated he wanted satisfaction but not a hurried solution.

After awhile, I reasoned I could remove the dimple, pound out the dent and then verify there was no damage done by asking the customer if he would test fire the repaired barrel to compare with the others. This comparison would be the litmus test to see if I had pulled this off. The alternative would be to make it right with a replacement barrel. If the repaired barrel shot as well as the others I could feel such a repair as this was a success. The customer agreed. Now I was faced with actually removing the dimple.

I first assessed the depth of the dimple. The barrel is in 7mm caliber and measures as groove diameter 0.284", lands 0.277", rifling depth 0.0035", six groove rifling, groove width about 0.050" and outside diameter 0.553". The maximum mandrel diameter passed at the dimple was 0.272" as received for repair, so the dimple was 0.005" into the land diameter. My customer certainly had a reason for concern.

To remove the dimple, I used successive mandrels with gentle tapers on their ends to cold upset the bore back to land diameter. The mandrels are hardened and highly polished. Using a little high pressure lube on them, mandrels were inserted from the muzzle in 0.001" diameter increments to bring the bore back to 0.277" at the dimple. Now that the dimple was a flat top and at land diameter. Did I mention the dimple was almost completely located in a groove? How to remove it from the groove is the next challenge.

The ramp is a Williams Gun Sight Company unit installed very close to the muzzle, making the threaded hole and dimple very close to the muzzle, too. If I were to reach the dimple with an upsetting tool (broach) from the muzzle it would have to extend into the barrel just under one inch. I fabricated a steel broach slightly less wide than the 0.050" groove width with a thickness of about 0.042" for strength.

Having a broken Browning A-5 flat spring already heat treated to spring temper, I carefully ground it to size and shaped with an anvil or ironing part only at the extreme end, reducing the dimensions elsewhere so as to not alter the rifling grooves on the way in. This is hard enough to survive the abuse of being used to remove a dent in the rifling of a modern chrome-moly barrel, which is likely not harder than 30-35 Rockwell "C" scale hardness.

The broach was used with successively larger undersized mandrels and pushed in the dimpled groove until the dimple is upset back to groove diameter. Soft lead slugs driven through the bore before and after the repair and inspected with magnification verified the effectiveness of the repair. I trusted the sharpness of the adjacent rifling lands to suitably guide the broach the one inch distance to the dimple with care taken not to not round off the two adjacent rifling lands as the broach travels in.

No account was taken of the rifling twist in this exercise, but given the short distance traveled the rifling twist effect is small. A light touch is definitely called for. The broach was driven with a small hammer as barrel steel is being upset and moved a full 0.0035" of dimple height in the groove. This was done using successively larger mandrels to back up the broach and push it incrementally into to dimple until it was fully reduced to zero height in its groove.

This is easily determined by feel as the broach had the raised ironing part only at its end. The dimple only showed as a small mark of no height in the groove after. Carefully measured soft lead slugs driven through the bore after the work measured the same 0.284" diameter for each of the three pairs of rifling grooves, verifying the dimple was reduced to zero. I measured this with a thin-jawed dial caliper.

Note reduced to zero dimension is not really the same as "gone" as it was still possible under magnification to see which groove had the dimple and observing the rifling groove scratch marks on the lead slug showed differences. A lead lap could have further smoothed this out if desired but my goal was that the dimple no longer could affect accuracy.

In test firing, the repaired barreled action was assembled as a complete rifle together with one of the others, both as identical as could be made using the 100+ year old actions and shot for comparison. Five shot strings for record indicated each barrel shot substantially the same. My customer appears satisfied with these results, as am I. AG