There's no trumpet mouthpiece which is 6,3 mm inner diameter. In general the trumpet mouthpieces goes from 15 to 18 mm and that's ruffly. Probably you were referring to Bach mouthpieces. In their charts it's shown that 3C is 16.3 mm. The problem is that they're comparing as an equivalent of Schilke 13C4 which is shown respectfully in their own chart as 16.89 mm inner diameter. In reality the 3C is slightly bigger than that. The different manufacturers measure the inner diameter at different depth. And there is another thing that we must take as an account. The different rim shape and inner edge give us totally different all round feedback about the size of the mouthpiece. With that instrument in your hand you'll never measure a mouthpiece precisely. Greetings from Bulgaria!
There's no trumpet mouthpiece which is 6,3 mm inner diameter.
In general the trumpet mouthpieces goes from 15 to 18 mm and that's ruffly.
Probably you were referring to Bach mouthpieces. In their charts it's shown that 3C is 16.3 mm. The problem is that they're comparing as an equivalent of Schilke 13C4 which is shown respectfully in their own chart as 16.89 mm inner diameter.
In reality the 3C is slightly bigger than that.
The different manufacturers measure the inner diameter at different depth.
And there is another thing that we must take as an account.
The different rim shape and inner edge give us totally different all round feedback about the size of the mouthpiece.
With that instrument in your hand you'll never measure a mouthpiece precisely.
Greetings from Bulgaria!