my article about choosing a cornet mouthpiece which includes a discussion of balancing out the resistance I thought I would share my knowledge of cornet mouthpiece throat sizes. I have converted the sizes to US drill sizes for easier comparison. For a description of what these sizes mean see Jim Donaldson's very informative article here.
Throat size is one element in determining the resistance of the system (the others being instrument bore size, mouthpiece gap and embouchure aperture). Its possible to compensate against a very resistive instrument by using a mouthpiece with a larger throat (bore) size or against a very open instrument by using a tighter mouthpiece.
Standard - #27, 3.66mm
Megatone - #26, 3.73mm (larger throat designed to overcome the tight feeling that results from the larger mass of the megatone mouthpiece)
Bach used to (maybe still) do custom mouthpiece work and I have seen a 5V (very deep V cup) with a larger #20 throat that was stamped "20" in the factory.
Standard line - #26 (except models with "A" backbore which are #27)
Schilke now make deep V shaped "symphony" models. I do not know what throat sizes these use.
Models without letters - 4.6mm #15
except: 2 which is #14
B models - 4.3mm #18
S model - 4.1mm #20
These mouthpieces were originally designed for the old Imperial and early Sovereign cornets which had more resistance than current models. I feel that they tend to encourage over blowing in modern, freer blowing, instruments. Wick seem to have recognise this issue by making a 4 ½ which is identical to the 4 but with a narrower #16 throat.
BBC (Brass Band Cornet) - 4.07mm #20
VC (Vintage Cornet) - #18
DC (Deep Cornet) - #25
Yamaha (short shank)
11E4 - 4.2mm #20
16E & 14E - 3.98mm #22
David King - 4.60mm #15 (presumably influenced by the Wick standard of larger throats)
Long shank (standard) #27
(Warburton make #23 mouthpieces as a standard option so these are also in circulation but marked "23")
Short shank with "BC" tops for UK market #20 (unconfirmed though as I have not seen one personally)
The shape of the cup of a mouthpiece also affects resistance with flatter, shallower cups generally being more resistive. You can test the relative resistances of two mouthpieces by running a constant stream of water into them from a tap and observing how much it will take before the cup starts to overflow. Its surprising how much some apparently small mouthpieces will take (try comparing a smaller Jet-Tone trumpet mouthpiece with a standard Bach 7c - the apparently small Jet-Tone will handle as much flow).