The standard for calibers doesn't seem to be as exact . i have 50 cal. muzzleloaders that vary by a couple of 1000's. Some a 50 cal sabot are almost impossible to push down while there are some that the same sabot is almost loose. The tight fit is important for accuracy. A loose fitting sabot allows gases to blow by and reduces accuracy. A squib shot is only necessary if your sabot doesn't fit tight in the bore. Most modern muzzleloaders seem to be getting tighter tolerances so a squib shot usually isn't necessary, all depends on the gun and what you are trying to shoot