Aside from the environmental factors affecting the hearer there are gun/ammunition factors at play.
1) subsonic -v- supersonic ammo. The De Lisle carbine was famously quiet because of the use of subsonic ammo and a large suppressor
2) the mechanical noise of the action. Manually operated guns have significantly less noise than auto and semi auto actions. Allegedly the suppressed Sten gun had more mechanical noise than bullet noise.
3) the amount of charge in the round. More gunpowder = more noise
4) caliber. Large caliber guns tend to have a lower note (more ‘boom’ than ‘crack’). Lower notes travel further than higher notes, but higher notes are more noticeable.
5) the type of powder, although this should just be broadly delineated as black powder -v- smokeless powder. Faster burning powder makes a louder higher pitched noise.
6) the size/condition of the suppressor.