That is correct. Missouri Bullet's sells slugging kits by caliber online, and you might prefer one of those for your first try at the practice. An OD thimble micrometer will measure more accurately than calipers (which some YouTube video authors use) and also usually have a vernier scale to resolve tenths of a thousandth of an inch, which let's you compare slugs to spot a constricted places in the bore other issues.
Bullet data is usually interchangeable as long as you stay with the same weight and general shape and, most importantly, type of construction. But ignoring any of those can result in up to 30% difference in pressure.
Military rifles, especially those made in wartime, can have more generous tolerances than we expect. The US 30 Cal bores during WWII had a tolerance span of three thousandths, with bore diameters from 0.2985" to 0.3015", and groove diameters from 0.3075" to 0.3095". Most are closer to the middle of the range, but you do have to slug to be sure of what you've got.
Bullet data is usually interchangeable as long as you stay with the same weight and general shape and, most importantly, type of construction. But ignoring any of those can result in up to 30% difference in pressure.
Military rifles, especially those made in wartime, can have more generous tolerances than we expect. The US 30 Cal bores during WWII had a tolerance span of three thousandths, with bore diameters from 0.2985" to 0.3015", and groove diameters from 0.3075" to 0.3095". Most are closer to the middle of the range, but you do have to slug to be sure of what you've got.