I have long felt it is important to know the critical chamber and barrel dimensions of all my guns. This, to enable development of optimum handloads and so as to track deterioration in those dimensions with time. Tracking the amount of throat and rifling erosion, allows that adjustments can be made in the handloads so they remain as optimum for the full life of the gun. Measuring the various dimensions of a fired case can provide approximations of the chamber dimensions, but trying to determine the actual dimensions in this way is questionable at best. There has always been further question about how to accurately determine the gun’s critical internal dimensions, especially those inside the chamber without sophisticated measuring devices. Over the years, I have fashioned several very simple “home-made” tools that when used along with conventional calipers and a high quality micrometer allows very precise measurements of the most important of the chamber dimensions. In other words, the shooter can accomplish at least a partial “blueprinting” of his gun. I will describe those tools here and also show how the advanced shooter can make them in a modestly equipped home workshop and in turn use them.
When talking with other shooters about knowing the gun, the first question always is - what do I need to know about my gun and why do I need to know it? The accuracy buff or the advanced handloader can tailor his ammunition to take best advantage of the makeup of a gun he already has if he knows the dimensions of its working parts. Usually, the shooter who analyses his gun, and has made it to shoot as good as it can, will then see something he thinks would make it better. At that point he may decide to buy a custom gun in order to get additional features he thinks will allow him to shoot better. His first task then is to specify some critical dimensions for the custom gun. In turn, he and his gunsmith may discuss the use of a high quality barrel blank or a given twist rate along with his other specifications. Finally they will settle on several things that they both feel will give him the performance he is looking for. Then it is very important to confirm all the specifications in writing, to the gunsmith before the work is done. But, as part of the specifications, it is necessary to tell the gunsmith how much tolerance in the specifications is acceptable. It is customary to state these tolerances in +/- thousandths of an inch from the specified dimension. The overall length of the chamber might be specified at 1.400” +/-0.005” for example. The gunsmith’s price for the gun may well depend on the span of the tolerances specified, as that will indicate to him the precision level he must perform to.
When that custom gun is delivered, what then? Will the shooter assume the gunsmith has built the gun strictly to his specifications? Well, let me assure you – If he wants that gun to perform in his special way, he had best not just assume it to be done the way he specified it to be. And yes, that is a whack at custom gunsmiths, but not without a reason. In the past several years as I have become more and more interested in accuracy shooting, I have owned nearly a dozen “custom-built” guns, produced by several different gunsmiths. To date, the only gunsmith who has produced the gun exactly as I have specified is the one we all know - Mike Sirois of OTT. All the others have missed on at least one and often several dimensions. In one case the gunsmith totally ignored my specifications. In fact he missed every one of my chamber specifications and apparently felt I wouldn’t notice. He then expected me to accept the gun with a chamber he had done a sloppy job of cutting to “standard dimensions”. He did this even though I had told him in the beginning that I know how to check the specifications and I would. He insisted that I shoot the gun as “the proof is on the target”. I replied that the proof of the first step, the one that determines whether I accept his product, is in whether he had met my specifications, which he had accepted when I placed the order. My question to him was – “how can you call yourself a custom gunsmith if you don’t build to the customer’s specification”? In the end, in spite of the fact I had prepaid for the gun, I refused to accept the shoddy work and got a full refund. OK, so that is one way that being able to document critical dimensions of a gun can pay off.
On a more positive note, being able to document critical dimensions of your gun will enable you to closely tailor your brass and handloads such that there will be the potential to eliminate or minimize common errors associated with that endeavor for high performance. In fact, with the ability to produce that higher quality of handloads, the shooter will think more deliberately about his loading methods and in general will be able to raise the quality of his handloads and his shooting to that next higher level.
Mandrels: The first tapered mandrel consists of a short piece of rod a little larger diameter than the chamber neck. A short piece, 3” or 4” long will work. Square off each end of this rod by chucking it in a drill press, then, lay a file flat on the surface of the drill press table and hold it in place. With the drill press turning at about 200 rpm, bring the rod down onto the file where the file will cut the uneven ends of the rod and make it square and flat. Chuck this piece of rod in the drill press again and with a file “turn” one end to have a shallow taper with the small end of the taper to be 0.010” to 0.020” smaller diameter than the chamber neck. The portion of the taper that is a smaller diameter than the chamber neck must be shorter than the length of the brass neck. Next, with sandpaper having about 220 grit, sand the tapered surface lengthwise so the small scratches from the sandpaper are lengthwise of the tapered portion of the rod.
A second tapered mandrel to fit into the throat can be made in the same manner by using a short piece of brass rod a little larger diameter than the throat of the gun, but smaller diameter than the neck. In the same way, turn a shallow taper so the small end of the taper is only 0.010” or so smaller than the throat diameter and the portion of the taper that is smaller than the throat is shorter than the length of the freebore. Again, with sandpaper having about 220 grit, sand the surface of the taper lengthwise the same as was done with the first mandrel.
Slugs: Chuck in the drill press, a short piece of brass rod that is a small bit larger than the neck diameter of the chamber, and with a file flat on the table of the press, square off one end. Then with a file turn the diameter of about a ½” section on the end to be just one or two thousandths smaller than the neck diameter of the chamber. Next, cut off a section of this turned rod about 3/8” long. Put the squared end in the drill press chuck and square up the other end with the file on the drill press table.
Flat-tipped jag: From an old gun cleaning kit, take the round jag and square off the end to form a positive, square ended stop for the cleaning rod. Using the file on the table of the drill press can do this. If you will be working with a gun having a bore larger than 30 caliber, a larger diameter replacement jag/stop can be made from a piece of ¼” brass rod. In the same way as before, square off one end of the ¼” rod. Then, place the opposite end in the drill press chuck. With a file, turn a 3/8” long section of that end to form a smaller shank approximately 0.145” diameter. Then, with an 8-32 die, cut threads on the 3/8” long shank. It will now replace the smaller squared off jag at the end of the cleaning rod and can be used for measurements in the bores larger than 30 caliber as it will span more than half the bore diameter and can serve as a stop against a spire point bullet tip.
Stop collars: Cut two pieces of 3/8” brass rod, one inch long and square off both ends of each using the file on the drill press table. Center punch one end of each piece. Drill a 13/64” (0.203”) hole (the same diameter as most small bore cleaning rods) lengthwise through each piece of 3/8” brass rod. It is easier to drill this hole to be straight and centered if the drill is held in a drill press vise, pointed straight up and the brass rod is in the drill press chuck. Then the brass rod is lowered onto the drill and the hole is drilled all the way through the length of the one inch long pieces of brass rod. A brass or steel washer about ¾” diameter is then soldered on one end of each piece of brass rod. Since it is difficult to get these washers soldered on the brass squarely, they can be squared up using the file on the drill press table again. A securing screw can then be fitted to each stop collar by drilling and tapping a hole in the side of each brass section. It will be more convenient to use the cleaning rod and stop collars if the securing screws are a wing nut or a thumbscrew type.
Neck diameter: The diameter of the chamber neck can be accurately measured with the tapered brass mandrel that has an end diameter just under that of the chamber neck. Insert the mandrel into the chamber of the gun such that it touches the beginning of the neck lightly. That is the portion of the neck at the shoulder/neck juncture. Rotate the mandrel on its axis while it touches the neck and since it has been sanded lengthwise, where it touches and rubs the neck during rotation, the rotation will cause the surface of the mandrel to be burnished and shiny. That burnished ring can be seen in the picture on each of the three brass mandrels shown. Withdraw the mandrel and measure the diameter at the front of the burnished portion. That is the diameter of the neck and if the taper is sufficiently shallow, it is the true diameter on the neck. If the taper were to be too steep, the burnishing would occur partly on the radius that is machined at the shoulder/neck juncture and the burnish would indicate a diameter that is slightly larger than that of the neck. Therefore it is important to have a shallow taper.
With an accurate diameter for the neck of the chamber, the handloader can now turn the necks of his brass cases to be a specific diameter. I have experienced the best accuracy from most guns when the diameter of the brass case neck with a bullet seated is 1 ½ to 2 thousandths smaller than the diameter of the chamber neck. Without an accurate measure of the neck diameter, the handloader is left to either take the gunsmith’s word for the diameter, or guess. When you measure, you don’t have to guess.
Overall chamber length: Now that the diameter of the neck is known, it will be possible to fashion a brass slug that is just 2 or 3 thousandths under the diameter of the neck. After this slug has been fashioned to the correct diameter, a measurement of the overall length of the chamber can be made. Remove one stop collar from the cleaning rod and insert the rod into the end of the barrel such that the squared jag touches the breech face. Slide the remaining (first) stop collar against the muzzle and secure it in place. Remove the rod and drop the brass slug into the chamber such that it settles into the neck with a flat end forward. Replace the second stop collar onto the cleaning rod and insert the rod back into the bore. With a pencil or small brass rod, hold the slug against the end of the chamber neck and touch the other end of the slug with the stop jag on the cleaning rod. It is possible to feel the slug from both ends with the rod and the pencil. Now with the slug fully forward and the rod against it, secure the second stop collar in place against the muzzle end of the barrel. Remove the rod and measure the overall distance from the front of the second stop collar to the back of the back of the first stop collar. The distance needed here is from the front of the second stop collar to the front of the first stop collar, but since that is a difficult measurement to make, it is determined by subtracting the thickness of the first stop collar from the overall distance. That measurement is the distance from the breech face to the end of the chamber, and it is an important one.
The reason the length of the chamber is importance is that most gunsmiths let their neck reamer run long and make the throat length dimension very long – sometimes as much as 0.050” longer than the stated “maximum length for the brass”. They do this to protect us from ourselves so we can’t let our brass get too long. But, for high quality handloads, the overall length of the chamber should not be more that about 0.020” longer than the brass case. When the chamber is too long, or when the brass is too short, and the round is fired, the bullet will obturate (swell) into the space left between the end of the brass and the beginning of the throat. This can result in deformation of the bullet and in turn to have abnormal flight characteristics, but more importantly can cause the breech pressure to go very high due to the obstruction caused by the obturation of the bullet into the void at the end of the brass.
Throat diameter: In a manner similar to that for determining the diameter of the neck, the handloader can determine the diameter of the throat. This is also an important dimension because if it is oversize, the bullet will not be fully supported during the time when it is entering the rifling and will be allowed to cock partially off-axis. This of course will lead to the bullet having a built-in yaw.
Maximum overall length of loaded rounds: By the same method used to determine the overall length of the chamber, the maximum overall length for a loaded round can be determined. This can be done by dropping a bullet into the chamber and determining the length between the breech face and the tip-end of the bullet. But, most bullets, both the lead-tipped and the plastic-tipped ones, vary slightly in length from one bullet to the next. However, essentially all bullets of the same type and weight (e.g. all Hornady #3031, etc) are the same length from their base to the portion of the ogive where a Comparator makes contact. So, the maximum overall length of a loaded round including the dimension of a comparator is a repeatable dimension regardless of any variability in the bullet’s length.
The maximum overall length of a loaded round is a dimension that needs to be monitored with time. This is true because the rifling will erode with repeated firing and after some time a bullet, when dropped into the chamber, will extend farther. So, if the best accuracy has been obtained with the bullet set to touch the rifling, the overall length will need to be adjusted as the rifling erodes. For example, for the XP-100 tested and shown in the earlier paper about bench shooting, began with a new barrel and during 500 shots while the gun was being tested and loads were developed, the rifling eroded so as to allow the bullet to extend 0.007” farther into the throat.
Rifling twist rate: The easiest dimension of all to determine is that of the rifling twist rate. Simply extend a cleaning rod through the bore from the muzzle end. Insert an extra large, tight patch into a slotted jag and draw it into the bore. When it begins to rotate, place a masking tape flag on the rod at the muzzle, and draw the rod on through the bore making sure the patch follows the rifling. When the flag has made a complete revolution, measure the distance it has taken to do it. That is the twist rate in inches per revolution.
Slug the barrel: It is seldom that the bore of a barrel will be oversized, but is worthwhile to know, if it is so. An easy way to measure the bore diameter is to “slug” it. This can be done with a soft lead ball that is 0.015” to 0.020” larger diameter than the bore. First, coat the bore with light oil to assure the lead ball, which is to be forced through the bore, will be lubricated. Then with a wooden dowel slightly smaller than the bore, and a small mallet, drive the lead ball through the bore from the breech end. After the lead ball has been recovered, measure its diameter with a micrometer.
Record the details: After having well documented the details about you gun, take the next step and record them for future reference. I have found it convenient to keep a small loose leaf “black book” with two or three pages for each gun I own. On these pages I have a drawing of each chamber, a complete listing of all the critical dimensions, a time history of those dimensions, all the details about forming cases and handloading that round along with recipes for all the handloads that perform well for each of the kinds of shooting I do with each gun.
So, in summary, when you know how to assess a gun, you can be more discriminating in your selections. Especially, when you pay the price for custom gun work, you deserve to get a product fully meeting your specifications. Then too, you are well justified in “holding the gunsmith’s feet to the fire”. If he fails to produce what you ordered and paid for. Following acceptance of that custom gun, when you have blueprinted it and know it, you are able to do a more sophisticated job of handloading and you will have the potential to become a better shooter as well.
If you think of something else that would work well in this area, tell me about it too.
Don