How Does It Work? Brownells Firing Pin Protrusion Gauge
By Steve Ostrem
The Firing Pin Protrusion Gauge is an extremely useful tool that no shop should be without. Like many gunsmiths, I’ve had made do with other methods, but years of relying on the depth gauge on my dial calipers was simply not enough. Not only is this way extremely less accurate than using the Firing Pin Protrusion Gauge, some jobs, like miking the protrusion inside the window of a revolver, are nearly impossible. With the Firing Pin Protrusion Gauge, measuring the firing pin protrusion of a revolver is a simple two-minute job. The same holds true for rimfire rifles and the Remington centerfire rifles with their recessed bolt faces. No more measuring to the pin, then to the bolt face, subtracting the difference and hoping the rim around the bolt is uniform in height. This handy little gauge definitely makes correct measuring simple and correct every time!
Now that you know why I like this product, let me tell you how it works. This tool is made of steel and consists of the body, the thumbscrew, and the plunger. The length of the body and the plunger are the same, and the ends of the body are stepped down. The smaller step is eccentric and the plunger is exposed on one side. This allows the gauge to be inserted into a .22 rimfire (or .17 rimfire) bolt recess and the plunger able to make contact with the offset firing pin. The larger stepped end of the tool is concentric with the plunger and is used for centerfire rifles, pistols, and shotguns.
Before using the gauge, you must measure its overall length. Mine measures .900” but they can vary by one or two thousandths. Make a note of this length, as it becomes the baseline for all future measurements. Actual use of this gauge goes as follows:
First, loosen the thumbscrew and set the plunger flush with the end of the gauge. Then, tighten the screw just enough to provide a little friction on the plunger. Next, do whatever is necessary to place the firing pin in its forward-most position. With the firing pin positioned as far forward from the breech face as it can go, loosen the thumbscrew and push the appropriate end of the gauge, (rimfire or centerfire), over the firing pin until it contacts the breech face. The firing pin will push the plunger back into the body the length of its protrusion. Then, all you have to do is tighten the thumbscrew to lock the plunger in place and measure the overall length of the gauge. This measurement, minus the original length of the tool gives you the actual firing pin protrusion. For instance, after using the gauge on my Smith and Wesson Model 27, the gauge measured .955”. I then subtracted the .900” that the tool measured originally. That left .055” of protrusion. (.955”-.900”= .055”) Easy huh? This amount of protrusion is about right for that particular gun.
For other firearms, the correct amount may be longer or shorter depending on the make and model. As a general rule, centerfire rifles and handguns usually measure between .050” and .060”. Shotguns are a little longer at .055” to .065”. Rimfire guns usually fall into the .028” to .035” range. However, inertia-type firing pins, the slanted pins in over-and-under shotguns, and many other designs will provide exceptions to these generalized guidelines. If you’re in doubt about how much protrusion is correct, try to find other examples of the same model to measure or try contacting the manufacturer for the specifications. Another good source of information is the series of shop manuals by Jerry Kuhnhausen. These books are great source of detailed information and specification for the many models they cover.
If you are curious about how our products work, drop us a line and let us know. In the subject line put “How Does It Work?” and we’ll sure look into it. Also, be sure to check out the Bench Talk and WebBench Articles and the Brownells Product Instructions area on the website. Lot’s of good info for many of your gun projects!