I deal with this all the time since Civil War arms (which I shoot in competition) are nearly always 3 groove. We use plug gauges to get the correct bore size since the groove depth is almost always .003-5. Next, forget anything in quality molds being "standard size". A good mold outfit will make what you want if you know the actual bore size. Moving on to my roller, it was 4 groove so common measuring techniques apply. My rifling was .015 deep, a 1:42 twist, and heavily eroded in the throat. A wave of Bobby Hoyts magic lathe cured that and now my 50/70 takes a .512 bullet and can still chamber.
Point is, there's no such thing as "standard" when dealing with original black powder guns. SAAMI didn't exist and manufacturers did what they felt best and wanted the purchaser to use only their ammo
Bore diameter is a significant measurement for muzzle-loading arms, such as the majority of CW-used types, since the bullet must be of a size which can be loaded from the muzzle without excessive force, particularly when fouled. It is true that many of them are 3-grooved, but the most common rifle-muskets (Springfield and Enfield) have progressive-depth rifling, intended to be .015" at the breech and reducing to .005" at the muzzle. It is true that many specimens are considerably different from the standard, though bore diameters are less variable than groove diameters are. The reason that bore diameters are more consistent, at least in those arms produced by the larger, established armories and contractors, is that the final diameter is established by reaming, and properly made, maintained and operated reamers do not cut much, if any, oversize. Groove depths, however, are variable because they are cut with rifling machines which can produce grooves of any desired depth, and the operators apparently either did not have suitable 'GO' and 'NO GO' gauges, or did not use them. I am certain that arms makers of the CW era and later could, indeed, hold tolerances to a thousandth of an inch, if proper quality control measures were observed. As to later military arms, specifically those of Springfield Armory production, bore and groove dimensions were specified and standardized to .001" limits, and these could have been maintained (I have examined and measured SA bore and plug gauges for Trapdoor barrels and found them accurate as marked to .0001"), again, had proper QC measures been in place. On the other hand, I have measured literally hundreds of Trapdoor barrels, both .50 and .45 caliber, and found them to vary widely from the specified dimensions for groove diameters (.515" and .460", respectively), and can only believe that lack of quality control could account for the variation. Bore diameters, on the other hand, were very consistent with the standard, for the reason given above. In fact, a friend and I have just had an article published in 'Blackpowder Cartridge News' on the subject of variations in Trapdoor barrels.
While muzzleloading arms using blackpowder are limited to use of bullets which can be easily loaded, and depend on the expansion of the bullet to fill the grooves for accuracy, breechloaders can and do benefit from use of bullets of full groove diameter, and, while blackpowder can cause a somewhat smaller bullet to 'slug-up' to fill the grooves, I have never found this to be possible with any smokeless propellent, and no reason to think that bullets of less than full groove diameter shoot any better, if as well, with either class of propellent.
PRD1 - mhb - MIke - barrel maker, retired