Each plane has different filenames for gunsounds.
Gunsounds have the following names: 30cal, M2, 20mm, 30mm, and 40mm. 30cal is used for anyting around .30 calibre (7.62mm, 7.7mm, 7.9mm, .303, .30...), and M2 is used for anything around .50 calibre (12mm, .50...). 20mm, 30mm, and 40mm are just that.
The default filenames have no numbers. For example, the Spitfire Mk. I has eight .303 guns, so it's "30cal, 30cal_2, 30cal_3, 30cal_4, 30cal_5, 30cal_6, 30cal_7, and 30cal_8". There is never a "_1" suffix.
Use different filenames to see which sounds work where.