Trigger Work: How Good Is Yours, Really?

As an old-timer, I have listened to people discussing and comparing pistols and trigger jobs for many years. Phrases like "buttery smooth", "clean break", "crisp", "no creep", "pre-travel", "over-travel", and "short reset" come to mind. Double-action and single-action trigger pull are others. For the most part, buttery smooth, crisp and other subjective and relativistic terms are hard to quantify, as what may be acceptable to one person may not make the mark to another.

I came to gunsmithing with a machine design engineering background, so terms like smooth, crisp, and other subjective terms that cannot be quantifi ed are good for open discussions and debates, but leave little to offer when a client’s pistol hits the bench. So, you want to improve the trigger on a pistol. Where do you begin and how do you know you’ve achieved your goal, really?

I first looked at what could be quantifi ed when doing pistol work: trigger pull, both DA and SA where they apply, pre-travel, over-travel, creep, and trigger reset. Things like "smoothness" and "crisp" are not readily quantifi able but results are reflected in other work. For example, smoothness will be reflected in the DA trigger pull. Once you have polished and deburred trigger-group components, you should see "some" decrease in the DA pull. Again, not readily quantifi ed, but you should see some improvement.

The crispness or clean-break of the trigger will be reflected in the "creep", so if you reduce the sear-to-hammer engagement, the release should be more "crisp" and so on. To get to the finish line, you must first leave the starting gate. I must know from where I am starting to have something to compare with when I am finished. Enter the pistolrevolver fixture.

I designed and built a fixture just for measuring trigger functionality, again, keeping in mind the quantifi able parameters of total trigger stroke, such as, DA/SA, total SA The original vise clamp screw is also seen sitting above the vise. The mounting plate was made from apiece of 1/4-inch thick fiberglass and cut rubber mounting pads from an old tire inner tube. trigger stroke, pre-travel, break point (or point of fi re), over-travel, distance to reset, etc.

Starting with a very inexpensive drill press vise as illustrated, I proceeded to strip it down, square it up, add guide rods for the moveable jaw, refita larger diameter, fi ner pitch clamp screw and fit clamps and mounting plates so a pistol or revolver could easily be mounted to it for taking measurements. The purpose of the guide rods is to stiffen the moveable jaw so it doesn’t wobble and lose position. It is the moveable jaw that actuates the trigger when taking measurements.

Any unwanted movement will lessen accuracy. Two 1/4 x 2-inch long dowel pins were added to the vise, one to the fixed jaw and one to the moveable jaw for actuating the trigger when the vise jaw is moved. The original vise clamp screw, which is attached to the moveable jaw, was coarse threaded and very loose in the nut. I made a

fi ner pitch screw and rethreaded the nut so there was very little play. This again aids accuracy. The fi ner pitch screw increases the mechanical advantage, making it easier to move the jaw which is pulling the trigger when taking measurements. The mounting plate was made from apiece of 1/4-inch thick fiberglass. I cut some rubber mounting pads from an old tire inner tube to make the pistol more stable and secure when laying on the fiberglass plate.

I also cut several 1" wide x 2" long x 1/4"-thick strips of rubber that I stack for securing the pistol when clamped. They act to fi ll the gap between the grip and the clamp due to the different pistol grip width variations. It is hard to tell in the pictures but the clamp pivots on one of the screws. This allows it to swing so the clamp can be located anywhere along the pistol grip. The clamp is a toggle clamp readily available from hardware suppliers like Mc- Master-Carr Supply. Various sizes are available.

How the Fixture is Used The pistol is placed and clamped on the fixture with both dowel pins within the trigger guard. The pistol is located so the pin on the fixed jaw is against the trigger guard. It is then clamped on the fixture, using rubber pads and the toggle clamp. Once the pistol is clamped to the fixture it must remain in that position for all measurements. If moved, you have to start again.

I locate the pistol in the fixture such that the pin on the moveable jaw will contact the trigger approximately 3/4 inches down from the trigger pivot pin. This isn’t always possible but seems to work for most pistols and revolvers. Having afixed position for trigger contact allows you to compare data taken from similar pistols to check for conformity. I have found similar model pistols, especially of the newer varieties, to vary considerably.

With the new manufacturing methods, I was expecting more uniformity but it appears not to be the case. We are now ready to take measurements. I proceed by turning the vise screw until the dowel pin on the moveable jaw just visibly contacts the trigger. The pistol is mounted and "zeroed" by placing and clamping on the fixture with both dowel pins within the trigger guard.

Locating the pistol with the pin on the fixed jaw against the trigger guard, it is then clamped on the fixture, using rubber pads cut from inner tube to make the pistol more secure in the toggle clamp. This is the "zero" or start position. Originally, I would take a set of digital calipers, measure across the dowel pins and write this as the "start" position. Eventually, I modifi ed the fixture by adding a one-inch stroke digital depth gauge to it.

Now, when the moveable trigger-dowel contacts the trigger, I zero the gauge so "0.000" is my starting position for all guns. Once the start is recorded, I slowly turn the vise screw to bring the trigger through its various stages all the way through its full-stroke position. The accompanying chart is a possible scenario for a DA/SA pistol. Lines one through three covers the full motion of the trigger in DA mode. As you can see from the last column, the depth gauge makes life a little easier.

The fixture allows the movement of the slide without unclamping the pistol, so once the DA fi re position and full stroke are recorded I can move the slide and cock the hammer for taking the measurements shown in lines four and fi ve by turning the vise screw in the opposite direction.

The pre-travel is found by subtracting line fi ve from line four. Once the trigger stops its forward motion, I turn the vise screw and return the trigger to the reset position. I then continue to turn the vise screw very slowly and record the point of SA fi re, line six. Subtracting line four from line six is an indication of trigger creep. Normally, I perform lines four through seven a couple of times to check for accuracy and to make sure the pistol didn’t move while setting the slide.

Note that the numbers are increasing as the trigger is pressed, making the results counter-intuitive. There may be a couple of thousandths variation between repeated measurements for various reasons, but this doesn’t affect the overall results. A gross difference will indicate something has moved. I enter the recorded numbers in an Excel spreadsheet to calculate the DA stroke, SA stroke, SA reset, pre-travel, over travel and other parameters I am interested in.

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