Fitting Dovetail Sights
There is more to installing dovetail mounted sights than just pushing them into the sight cut in the slide. When fitting dovetail sights, the goal you are trying to achieve is to have a sight that fits flat against the horizontal cuts that make up the slide's sight cuts with as tiny a light gap as you can get and completely fills the dovetail with no gaps on the bottom or sides of the dovetail.
Before you can install the sight in a sight cut & dovetail, you generally have to fit the sight to the sight cut & dovetail. This is done by measuring the old sight, the sight cut and the new sight and making a plan to remove material by filing from those surfaces that are too large. You can't just start filing material from the bottom of the dovetail when the sight has upper abutting surfaces like a dovetail front sight, or my HD-001, 002, 004, 004.5, 005, 006, 007 & HD-008 rear sights.
There is something important that you need to know about machining dovetails - the dovetail cutters wear as they are used and they wear more at the relatively delicate tips than anywhere else. As the tips wear, they become rounded. This means that in your slide's dovetail, where the sides and bottom intersect, those corners are more likely to be a radius than a sharp corner. The same thing happens on sights that have a flat above the dovetail to one or both sides; you probably have a small radius on the sight where the side of the dovetail stops at the top and intersects with the adjacent horizontal flat surface.
If you ignore these radii and don't make some sort of allowance for them, you may wind up with a sight that will try to climb up on top of the radii as the sight is driven in, leaving a gap underneath.
Prepping the sight and slide
What I do to eliminate this problem is to file a chamfer (a small flat) on the sight's dovetail on the two bottom corners of the dovetail where the sides meet the bottom. A chamfer about .030" wide is usually plenty. I also file chamfers on the slide where the sides of the dovetail cut ends at their tops. I chamfer both the front and rear if it's a sight that has flats forward and behind the dovetail, like my HD-001, 002, 006. I chamfer just the rear on the HD-004, 005 or any Novak style cut and I only file a light chamfer on a USGI style rear sight like my HD-003 Retro. Obviously, it's only needed where you have an intersecting horizontal flat. One additional thing that I do on Novak sight cuts is to also put a chamfer on the forward edge of the flat where it meets the dovetail. I file this chamfer about 1/8" wide on about a 1 degree angle to the flat, which is very shallow. This usually is a help in getting the sight to sit flat in the cut.
Checking and adjusting the dovetail depth
Once you have the chamfers filed on the sight and slide (as appropriate), you need to measure the depth of the dovetail in both the slide and on the sight (skip this step on the USGI style) and write the measurements down. I use the depth gauge feature of my digital caliper, being very careful to hold it square and flat to get an accurate measurement. I check several times to be sure I get an accurate measurement. A plastic caliper that came in your reloading kit is not adequate for this job.
If your sight's dovetail is deeper than the slide's by more than ~ .001", then you should file off the difference from the bottom of the sight’s dovetail, leaving the dovetail on the sight just a touch deeper than the dovetail in the slide. If your slide’s dovetail depth is greater than the sights dovetail, then you must remove material from the top of the flat above the dovetail. I would advise making this cut with a mill.
If you are fitting a sight that has a cut with a rear pocket or windage section like a BoMar or LPA (my HD-001, HD-002, HD-006), you also need to measure depth from the main flat to that rearmost flat, on the slide and sight. If the sight has a greater measurement, you will need to machine off the difference, plus .001” for clearance. If the slide’s depth is greater, you are fine as there is not much way to fix that. I make my sights of that style to a dimension that is at the deepest of the design standard. Manufacturers that make this rear flat a different depth, tend to go shallower.
Measuring and fitting the dovetail width
The next step to make the front-to-back dimension of the dovetail on the sight fit the slide. Most likely, the sight's dovetail will be bigger than the slide, forcing you to make the sight’s dovetail width narrower. You can do this a couple of ways. You can measure the difference between the sight and slide and file the dovetail to fit, all the while being able to know how much you have to take off and being able to check your progress along the way, or you can file and test fit until you get there. I’m personally pretty uncomfortable removing material without knowing how much I’m trying to take off and how close I am to being to the correct dimension.
Since I install a lot of sights, I bought a dovetail measuring fixture made by XS Sights and available from Brownells as p/n 006-101-000. It costs $50. You need that good set of steel dial or digital calipers you used earlier to use with the fixture. This fixture will let you measure the sight's dovetail. The tool also contains a set of rolls that will let you measure the slide's dovetail. Instructions are included that have the formulas to convert the measurement across or between rolls into theoretical dovetail widths.
If I am replacing an existing sight, I remove the old sight and use the XS tool to measure the old sight's dovetail width, then measure the new sight's dovetail width. Subtract the new from the old and you have the amount you need to remove. If you get a big dimension, like more than .010" or so, you need to evaluate if you've bought the right sight for the cut and recheck your measurements.
You can’t measure a Novak cut without using rolls (rolls are ground steel gauge pins, usually the same diameter) and all you can measure on the sight’s dovetail is the actual tip-to-tip width, not the theoretical width to a point, which is the measurement you are trying to work to. Most any sight made already has some sort of chamfer on the sight’s dovetail tips, enough to give an inaccurate measurement, but not enough to clear a slide dovetail cut with a dull cutter. I’m not saying you can’t measure a sight any way other than how I described, but I don’t know of an alternative cheap way to do it.
Tips for filing the dovetail
With one exception, I would always remove material from the sight’s dovetail when fitting the width. If I needed for example, to take off .007" from the new sight's dovetail, I would hold the sight firmly in a smooth jawed vise using card stock to protect the sides of the sight. I would prefer to take the entire amount to be removed from one side of the dovetail, typically the rear side. On a Novak style sight, the front side of the dovetail is visible when installed and I like to keep one side of the dovetail as-machined, when possible.
I use a safe-sided parallel dovetail file in either 60 or 65 degrees, as appropriate (available from Brownells) to remove the material. When you are filing, be sure to not tip the file as this will cut a tapered dovetail and be sure to keep the safe edge snugly against the sight body's flat that is adjacent to the dovetail angle that you are filing because if you don't, you may change the angle of the dovetail. Cut a half-dozen strokes (only in the direction the file cuts, cleaning the teeth every few strokes) and measure your dovetail width again (or test fit if you aren’t measuring as you go). You can gauge your rate of progress and when you are within .002" or so of having the same width as your old sight, try a test fit.
If it starts, give it a couple of taps with a soft face hammer, watching for continued even movement of the sight into the sight cut. Once you get it started by 1/8” or so, remove the slide from the vise and hold it up to light. You should look all around for contact points where the sight is hard against the slide. If you see any, like at the front corner, or down in the rear pocket area, remove the sight, look at the bottom and file a stroke or two where the bind is and test fit again. If the inward movement binds up, stop and tap the sight out of the slide. Look at the sight’s dovetail and see if any material has been peeled up and if so, file it down flush. Take another file stroke or two on the dovetail and try again. I want my sights to start in about 1/3rd of the way to the center by using good smacks from about a 6 oz nylon face hammer and then have to switch out to a larger steel hammer and a soft aluminum or brass punch with the face wrapped in monofilament packing tape (that lasts longer, but masking tape is also good for a few licks) to knock it to center. Note that I am not using so much force that I am damaging the side of the sight or shaving metal from the sight’s dovetail.
On the USGI style sight like my Retro, both of the sight's angled surfaces show, so unless you are planning to refinish the sight anyway, you will leave the sides alone. You can remove material from the bottom of the dovetail until the sight fits, or you can open the dovetail in the slide to fit the sight. Note that if you measure and calculate the difference in dovetail width between the old sight and the new, that dimension only correlates between the sides of the dovetail, not the bottom. If you take material from the bottom, take about 60% of the difference from the bottom and test the fit. I machine this on my mill and if you cut too much past that 60%, you'll have gone too far. Also be aware that what you take from the bottom shortens the height of the sight. It's not a problem on Gov't Model sight radius guns, but by the time you are working on a Commander or an Officer's ACP, you will want to keep that rear sight as tall as you can to help you zero without having to use a really short front sight.