The stock throat in most trumpet mouthpieces (and many horn mouthpieces) is the smallest practical size, usually 27 or 28 for trumpet and 16-17 for horn. This used to be called a "pilot hole" and it was assumed that most players would enlarge it. A larger throat reduces resistance and can also balance a large or deep cup. Bore sizes in the range 18-26 work well with trumpets. Most hornists feel most comfortable with a throat in the 10-16 range. Larger sizes work best with very large cups.