Significant digits and decimals
Your caliper is set up to give you a reading to four significant decimals ( correct to 1/10,000 of an inch)
As this level of accuracy is only required by engineers in tool setting and tolerance determination, you can simply go to "3 digits or decimals of significance," so your .3115 becomes, by rounding up, .312 (Rounding rule is, if it is 4 or less, go to 0, if it is 5 or more, go up to 10.); so, also, .3160 becomes simply .316.
As to the importance of this level of accuracy in casting lead bullets or Bore slugging...very little importance whatsoever in a practical sense, unless your Barrel wear is such that accuracy is compromised unless you use oversized cast bullets to "Take up the slack"
Dimensions Land to land (ie, original BORE dimension) give an idea of the amount of barrel wear...the germans, czechs and Turks stamped the Factory Bore dimension on the barrel shoulder/shank to indicate to the Field Armourer the original factory bore diameter, so that wear could be gauges and barrels replaced as necessary...the Swedes did the same on their Butt discs of their M96 and M38 rifles, and Ag42 as well.
Groove dimensions can be variable for a number of reasons...one is that in many rifle designs, the Groove dimensions are notable larger than actual projectile diameters, by design, and not by wear ( see the original Mannlicher 8x50R M90 and 95 Rifles...Bullet .323, groove .329) The French had the same combination of .323-326 Bullet, but .325-327 grooves. And we won't go into the J/JS German controversy...
The other reason for wide diameter grooves is plain flame erosion, combined with corrosive and erosive (powdered glass) Primers, improper cleaning (or none at all), over a long period of time ( so-called" Sewer Pipes") a common feature of Ex-China Mausers and Arisakas.
Whilst the best Bullet to Bore corrispondence is a laudable goal, with cast bullet shooting, it is only necessary to have the bullet soft enough to Upset properly in the existing rifling (with or without gas check) and that the Bullet diameter is not "undersized" (less than groove diameter) nor so "oversized" that it swells the case neck and prevents cartridge seating in the chamber (Have had this happen with a Chinese Vz24 with a bore such that only .330 cast (ex Steyr Mould) would take the "rifling", but they had difficulty chambering, as the oversized neck would scrape into the chamber. One reason for "Wall hanging (or Film Gunning)" this particular rifle.
Regards, Doc AV