The Studio Master SP4 has an inner diameter of .637" or 16.18mm.
The SP, or Shallow Piccolo model combines elements of mouthpiece design that cover the best of both worlds. The cup is extra shallow and is shaped like an early baroque style mouthpiece with a tight throat area and a flat-bottomed, C style cup. This boosts the upper range while giving warmth to the sound. The bore on these models is a #25 (.144" or .378mm) which can help to release the air flow, especially for symphonic players who have been accustomed to playing larger bore sizes, so that this piece plays like a larger mouthpiece while maintaining the benefits of the shallower cup to color the sound and improve the range.
The trumpet shank is the same length as the LX models (3.240" or 79.76mm). The extra shortness of this length helps to tip the response of the overtones in the upper register.
The piccolo cornet shank has a bit tighter back bore than the Vacchiano piccolo models. This lends extra support for the upper register.