What type of gun would a mercenary use meaning 1 specific type of gun that they'd most likely use?

Answer 1

Mercenaries will most likely use a common military firearm of whatever time period they were in.

For example in the US Revolutionary war period the Hessian mercenaries hired by the British use single shot smooth bore muskets that were typical of the leading edge of military firearms of the day, but mercenaries today will use some model of full auto assault rifle having a fire rate of at least 600 rounds per minute.

Mercenaries often own their own firearms and may prefer and carry very different weapons even when in the same unit.