I grew up shooting Dad's .36 cal cap & ball revolver, and later the .40 cal rifle we built, and always thought of .50 cal as "big bore". Fast forward past my first .50 cal flint rifle I bought, then only a few months later got a deal on a used .54 cal cap rifle, and from that moment on, .50 cal seems significantly small.
Some folks at the range always consider the .50 cal rifles as 'big bore', are amazed when I load up the .54 cal's, and are awestruck when I showed them the balls for my .62 cal fusil.
One guy close to had a stroke when I pointed him to the guy shooting the .75 cal Brown Bess-type musket down at the end of the range.
I guess my perception of "big bore" is on a sliding scale that changes with everything I shoot that's bigger than the last, but it's not the bore diameter that would be significant in a new forum, but more of the 'Magnum Loads' in any caliber using high powder weights to propel massive slugs with the shoulder-crushing recoil.
Perhaps a new forum simply labeled: the Hi-Impact Forum would be appropriate.