Yep, confusing for sure.
People use the word caliber when the mean to ask what cartridge the gun is chambered for. You could have several guns that have a bore diameter of equal caliber, but they take different cartridges. .30-30, .308 Winchester, .30-06 Springfield, .300 Winchester Magnum, .300 WSSM, all may have the same caliber requirements for the bore and projectile diameter, but ehy all have different cartidge case shapes, powder volumes, etc.
But I guess asking, "What caliber is that?" is a bit more natural than "What is that rifle chambered for?"