Bottleneck rifle cases usually always work correctly as the sizing dies include an expander to open neck to proper diameter as part of the sizing operation.
With handgun cartridges, case neck wall thickness makes the difference. Sizing the case from the outside squeezes the case down. If the case wall is .002” thicker than nominal, that makes the inside diameter of the case mouth .004” smaller, making the gauge tight.
The gauge should be that same .001”- .002” smaller than the neck diameter. If it fits tight in your cases, you can simply reduce your gauge diameter.
Straight walled cases may give you some trouble with thicker brass, as the expanding is done as a separate operation. Expand the case before trimming or use a decapping die rather than the sizing die or you can remove the gauge from the cutter, clamp the gauge in a drill chuck and spin it. Take some 180 or finer grit emery cloth and polish down the large end until it fits into your case. This is only a thousandth of an inch or two, so it does not take much to remove it.
If you are having trouble getting the gauge into the cutter, take a hand drill and lock the gauge into the chuck. This is just to give you something to hold it with. Now screw the cutter on. They are made tight so they won’t work loose. Stop screwing the gauge into the cutter as soon as the shoulder touches the cutter teeth.
The first installation of the case length gauge to the cutter will require more effort because an oversize thread is used to provide a friction fit to the cutter.
