Originally Posted by
uscra112
Micrometer. If you have a digital caliper, that will be good enough for this job. But you might think about adding an economy model 1" micrometer to your tool set.
I can guess that Winchester wasn't too fussy about groove diameters because the soft lead boolits everybody used (usually patched) could be expected to "slug up", (i.e. expand their diameter), under the pressure of the powder explosion, to fill the grooves, no matter what size. As I wrote earlier, boolits for black powder rifles had to be smaller than we normally use for smokeless, because they had to fit into the barrel throat when it was choked with fouling, and they were soft lead, so they could expand as necessary. Rifle chambers were cut accordingly.
The only shooters who used boolits sized to right up to groove diameter were target shooters, but they cleaned their barrels obsessively after every shot.
Sizing that kind of case doesn't hurt it enough to matter. But the sized case mouth should be just small enough so that the boolit slides in easily, and doesn't fall out if you turn the round upside down and shake it gently. That's how I do it for my single shots. If your die is sizing smaller than that after the expander plug has done its' thing, you need a bigger expander, or maybe a different die. We won't know until you can provide us with some measurements.
In any case, you will be crimping aggressively to keep the boolits in place when they're in the magazine tube, and that will be the limiting factor for case life, not the sizing. Crimping the lip inward and then having it blown out on firing, and then flared for boolit seating, and then crimped inward again on the next reload, will cause the lip to start cracking long before anything goes sour from sizing.