Compared with a barrel band front sight ramp ( inner diameter of the barrel band equals the outer diameter of the barrel) the BRNO M21 ramp consists of a bushing (outer diameter equals the barrel diameter at the muzzle).
That means the muzzle part of the barrel is turned down to accept the bushing that is then soldered on.
That is done so close that it is nearly unseen and makes you believe that the barrel diameter at the muzzle is that what you measure there , when the front sight is on place.

Same construction is found on Rigby´s .350 rifles, but also on many military Mauser rifles.

Hope that will help You.