XM4 = inner diameter + .637" or 16.18mm
The XM Model has the longest overall length in the Studio Master Series. We arrived at this particular length from observing this particular trait among mouthpieces belonging to many lead players who had received their mouthpieces as 'hand me downs' from prominent teachers and mentors. The XM has a slightly tighter back bore than the VMS and VM models which can help to add just a bit of extra resistance to this otherwise very free blowing mouthpiece. The cup of the XM is slightly deeper than the XMS. Both have a #27 bore (.144 or 3.65mm).