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Here is another COAL gauge that will appeal to my fellow cheap skates!
Construction is simple and the cost for my gauge totaled $3.37 including sales tax! This gauge is a simple take-off on the old cleaning rod method of measuring COAL. In fact, your cleaning rod can be substituted for the steel rod shown below and you can save dang near two bucks! In the cleaning rod system, one placed the cleaning rod into the barrel and allowed it to rest against the closed bolt face. Then a mark was put on the rod and the same measurement was taken with a dummy round chambered in the rifle and another mark made on the rod. Then it's just a matter of measuring the distance between the marks. The cleaning rod system works ok but is not very precise given the possibility of errors in placing the marks and the actual width of the marks. There is also the problem of obtaining an accurate measurement between marks when there is no stopping point for your caliper. And then, finally, your cleaning rod is eventually a confusing array of marks that must be dealt with each time you do an OAL measurement. The ComponentsYou will need two collars as shown. These are obtained from a local RC modelers shop. The owner tells me they are used as bushings when installing wheels on model landing gear. These shown have a 3/16" inside diameter. The bushings are used in conjunction with a mild steel rod of 3/16" diameter. These bushings are sold in many different sizes. Obviously you would want a smaller size bushing and rod for a .22 caliber rifle and a larger size for a .45 caliber rifle. The three photos below show the components packaged, unpackaged and installed on the steel rod. Note that the bushings are sold complete with set screws and a hex wrench to fit them. While there are four bushings per package, we will only use two of them. This of course allows you to share with a fellow shooter and cut the $1.45 cost in half!! Yeah, I'm embarrassingly cheap.
Next you will need a dummy round as pictured below for the cartridge of interest. Each dummy cartridge and measurement will be specific to the particular bullet being loaded. In the case of this dummy round, a fully sized case has been cut with a hack saw to allow easier insertion of the bullet into the case, as well as easier removal of the bullet from the case when the measurement is done. This particular dummy round consists of a Swiss Schmidt & Rubin 7.5X55 case in a K31 rifle and a Hornady 168 grain A-Max bullet. The case of course is not primed!! The best situation for a dummy round is a case neck that falls somewhere between fully resized and fired. A fired case will allow the bullet to slide to freely in the case neck and you will have difficulty obtaining an accurate measurement. A fully sized case neck is quite tight and is usable but the removal of the bullet will be difficult after the process is complete. You can solve the problem in two ways. You can cut the fully resized case as pictured below, or you can go ahead and use the sized case as is and over the course of using it several times the case neck will gradually enlarge and become easier to use. Starting with the fully resized case and working it larger with use is probably the simplest solution, although you may have to pull a few bullets with pliers at first.
Using The ToolUsing the tool is very simple. Here's how it's done, step by step: Remove one of the collars from your rod and loosen the set screw on the other. Next slide the rod down the bore until it rests against the closed bolt face. Now, you tighten the set screw as in the picture below.
Remove the rod and chamber your dummy round with the bullet protruding well out of the case. Close the chamber firmly so that the bullet is pushed back into the case and is resting against the lands or tapered throat, whichever is the stopping point for your particular style of chamber. Now, put the other collar on the rod and slide the rod down the bore carefully until it contacts the point of the bullet. Be careful not to push the bullet back into the case! Now, tighten the set screw on the second collar as shown below.
Now remove the rod from the bore. I'm sure it's quite obvious by now that you are going to arrive at the COAL by measuring the distance between the two bushings, but there is one step that you must remember to do. That is, you must measure the thickness of the bushings. The COAL will be the combined measurements of the thickness of one bushing and the distance between the insides of the two bushings. So, as shown below we measure as follows. Remember, that you need only measure the thickness of the bushing once and that measurement will become a constant in all your future uses of that gauge.
End result here was: Bushing
thickness:
.220 A perfectly legitimate question to ask is; "Why not just eject the dummy round and measure it you dufus?" And, in the case of this particular cartridge and dummy round, it indeed did yield the same results! So, why do it this way? Well, first off, the K31 rifle has a very short and "blunt" throat. There is very little taper to the lands and the bullet in the dummy round is stopped by the lands without any gripping of the bullet in the lands taking place. Consequently, when the dummy round is ejected, providing there is enough tension holding the bullet in the case neck, the bullet will not be partially pulled from the case during ejection. Nearly all chamber throats have more taper to the lands, or the bullet slides into an unrifled taper in the chamber. In both cases, it is easy and typical for the OAL of the dummy round to change during the ejection process, or, equally common, the bullet is pulled from the case and remains stuck in the chamber requiring the process to be repeated after knocking the bullet out of the throat. I have taken measurements off dummy rounds as many as five times and gotten different measurements each time!! Email me to let me know what you think of this gauge or if you have any suggestions for improvement. Hope you find this useful in at your loading bench. |
