Here's a simple method of comparing mouthpiece gap:
1. Place your mouthpiece into trumpet #1 and mark the end of the receiver onto the mouthpiece shank in blue.
2. Repeat in trumpet #2 and mark in red.
3. Place a toothpick into trumpet #1 and mark the end of the receiver onto the toothpick in blue.
4. Repeat in trumpet #2 and mark in red.
5. Measure the lengths of both blue marks on the mouthpiece and toothpick and subtract the smaller number from the larger of the two. This is your gap measurement for trumpet #1.
6. Repeat for both red marks to figure the gap for trumpet #2.
7. Compare both numbers keeping in mind that they represent the difference in slotting and flexibility in each trumpet.
Why is mouthpiece gap important?
If both numbers are exactly the same you can objectively compare the playability of each trumpet when using the exact same mouthpiece. It is not possible to compare two trumpets objectively if the gap setting is not exactly the same on each receiver.
Mouthpiece gap will affect the resistance, slotting and flexibility of any brass instrument. Every player prefers a different gap setting that compliments their personal playing preferences and habits. The acceptable range of gap is usually between 0 and .150 for most players. If you discover a large discrepancy in gap from one horn or mouthpiece to another, then you may consider having your mouthpiece or receiver adjusted professionally or consider a Harrelson AGR (Adjustable Gap Receiver) be added to one or more of your horns.
Large gap = more resistance, which translates to a tighter feeling horn. Large gap is associated with very defined slots and decreased flexibility.
Small gap = less resistance, which translates to a more open feeling horn. Small gap is associated with loose slots and increased flexibility.