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I was always of the understanding that smallbore is 22RF, and perhaps now other rimfires , and big bore is any centrefire. I may have been mistaken all the years. (eg. smallbore rifle shooting is 22RF)
I don't think there is a specific definition for "big bore" - I think it is more a matter of personal opinion (would make a good poll). I have always considered the .338's and up big bore. .308Win through .300 Win Mag is pretty much the bread and butter of my shooting.
I think here in N.A. the .375 H&H is usually thought of as the beginining of the "big bores", but in Africa, where the .375 & 9.3x62 are considered "medium bores", it's .40 and above - but usually not including the 45-70; rather the classic dangerous-game calibres, .404, .416, .458, .470 NE etc.
Not sure why this difference exists, but it may be that here we have little need for calibres above .375 and .458, so they are at the top of the heap. But in Africa, they have another whole world of heavy artillery above the .375, from the .404 through .500 NE and 505 Gibbs up to the .577 Nyati or Tyrannosaur and .600 NE.
Anything below .400" is small bore, .400 -.458 is medium bore, above is big bore, if i remember correctly, this is the old African def. for English rifles.
For many years the designation for competitive target shooting was small bore,(22) and full bore, usually meaning the 303 British.
For casual designation, just take your pick and call it as you like.
Big-bore is relative. Carbines chambered for pistol cartridges have a fair sized hole in the barrel.
The definition of a heavy rifle is over .400, over 400 grains, .300 or higher sectional density, energy of 5000 fps and nominal velocity of 2150 fps or higher.
I can live with the over .40 is a big bore, but Ben got it right; big bores exceed .45 caliber, everything between .30 and .45 is a medium, going by the old English vernacular, and everything smaller than .30 is a small bore.
to me small bores are the deer rifles .22-.25 + pistol cartridges regardless of bore size, the mediums are the elk rifles .264-.366 + .444 marlin and 45/70 etc, and big bore is bigger than the .375 H&H in diametre, weight, and energy. i cannot decide if the .375 is a medium or big.