Mill Sight Adjustment

A tip from gunsmith Patrick Sweeney. A neat trick you can perform with a mill is to adjust front and non-adjustable rear sights.

Say your pistol is hitting low and you want to adjust it. The calculation involves four figures: The amount you need the bullet moved on the target, the distance to the target, the sight radius of your handgun, and the amount of correction of the sight blade.

The ratio of the bullet correction to the distance to the target is equal to the ratio of the sight correction to the sight radius. For example, let’s say a pistol with a sight radius of six inches is hitting two inches low at 25 yards. Convert the yards to inches and we have 2/900 = C/6, where “C” is the correction to the front sight. Multiply both sides by six and we get 6 X 2/900 = C. Thus we find that our correction will be to mill 0.013 inches off of the top of the front sight.

Another way to do the same thing is what a gunsmith friend of mine does. He checks all handguns with a sight-in distance of 28 yards. Why 28 yards? Because it is close enough to 1,000 inches (1008 to be precise) to make the math easy. Simply, any error on the target, in inches, times the sight radius, is his correction on the sights. A one-inch error at 28 yards with a six-inch sight radius means 0.006 inch of correction.

How to make the correction: Clamp the slide in your mill vise, level it, and then dust off the top of the front sight by the calculated amount. De-burr, cold blue and you are done.

If you are hitting too high, and your fixed rear sight has enough metal, you can do the same thing to the rear sight with one extra step. After you have dusted off the top of the rear sight with your end mill, switch to another end mill the same size as the rear sight notch (usually .125 inch) and deepen the notch by the same amount you had just lowered the top of the rear blade. If you didn’t do this, the notch in your rear sight would be shallower than it was before you started. When you de-burr the edges of the rear sight, be sure to slightly round the corners on the outside, so you won’t cut your hands handling the pistol. Cold blue the exposed steel, and you are done.

You can do the same job with a file, but you must be very careful. Even the slightest tip in your angle of filing will leave the front sight top angled. A visible angle on the top of the sight makes aiming difficult. If you plan to file, file to a depth short of the required depth. Then carefully, and with the finest file you have, file down to the required depth. Check your work regularly, to make sure you are filing evenly.

For more detailed advice on adjusting pistols and revolvers, check out Gunsmithing Pistols and Revolvers, 3rd edition from Gun Tests in the Books section. Gunsmithing Made Easy: Projects for the Home Gunsmith is also available and has additional useful information. Both can be found online at Gun-Tests.com/subscribe

Read more in our December 2014 issue. Back issues are available.

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