Lets get this question of BORE size settled.
The BORE of a rifle or shotgun is the original diameter of the the drilled and reamed hole in the barrel BEFORE
Rifling or Choking
It is also the universally accepted means of defining the Calibre of a Military Firearm.
(Calibre…thought to derive from the Latin Qua Libra meaning " how many pounds " …What Weight? Of ball; alternative derivations say it is an Arabic word relating to diameter or weight?)
Anything else in a rifled barrel is GROOVE DIAMETER
Which is NOT necessarily BULLET DIameter.
ESPECIALLY where 1880s–1890s Cylindrical Bullets were concerned, just like their Lead BP Predecessors were concerned.
Cylindrical jacketed bullets were made UNDERSIZED
Wrt GROOVE diameter for several reasons…jacket friction, metallic fouling, possible powder fouling (with semi smokeless and early compositions of smokeless)
They utilised the BASE UPSET OBTURATION system, where a concave lead base in the jacket on firing, expanded to fill the Grooves and thus give a gas seal.
( Minie style).
The best example of this is the M1891 7.62mm Mosin Nagant heavy ball bullet…Original drawings (Russian) show a bullet with a max. Shank diameter of .308", with an average .306" for most of the cylindrical part, and a .250" diameter conical impression in the base lead core of the bullet.
Imperial Russian designs in Inches.
Given that the BORE of the 7.62 MN is .300" ( trelinaya), and the GROOVES average out ( Given Russian production) at .310 to .312 on New rifles…the system seems to have worked fine.
The same applied in theory to the Gewehr 88. BUT with a BORE 7.9MM( .311) WITH 8.05 GROOVE AND A 8.05 BULLET…hence wear, barrel failure, overheating, etc.
Then rifling Deepened, 8.15mm( .320-321");, barrel and receiver steel changed and better controlled ( “n.m.”)
Bullet diameters varied with manufacturers, but Military
Bullets remained in the .316-318" diameter area.
Civilian barrels and projectiles were a totally different matter. BORES WENT DOWN TO 7,8, EVEN 7,75, AND GROOVES WERE .315-319" (many variations).
Then the 1903-5 change to the spitzer flatbased bullet…no more need for base upset, as the contact surface area of jacket to barrel was much reduced, the Bullet was made to approximate a new Groove diameter…8.2mm or .323" the S barrel…to match the S diameter Spitzer bullet The actual average Groove diameter was .3225" but rounded to .323" for tolerance and convenience. Gew 98 barrels are marked with BORE dia. for armourer wear gauging (7.90 to 7.94) at throat and muzzle.
Now the older Gew88s were found to be able to fire the S-munition, if the larger diameter case neck was allowed for in the chamber, compared to the smaller.
Diameter Gesch.88 and P88 case.
Once this neck and throat relief was done, the Gew88 could safely chamber and fire the S-munition.
The BORE was 7.9-7.92mm, same as for the G98, and the Grooves of the improved G88s were .320-321", and the shorter S bullet could draw down to the tighter groove diameter safely…a fact shown in WWI, when Austria rechambered 7.62 Mosins to Fire 8mmAustrian ammo without any ill effects!!!. ( DRAWING DOWN A CYLINDRICAL .324" BULLET TO A .300 BORE AND .310 GROOVE…QUITE A REDUCTION COMPARED WITH THE GEW88 S RIFLES!!!
Those are the basics leaving out misconceptions and misinterpretations and what may or may not have occurred between 1888 and WWI.
K.I.S.S. Our Sgt-major used to drum into us at Officers