Air Rifle

The air rifle was a protoype firearm utilized by the Assassin-turned-Templar Shay Cormac, during the 18th century.

A prototype weapon developed by the Templars, the air rifle propelled projectiles by means of compressed air, in contrast to widespread flintlock pistols and muskets, which used a propellant charge. While incapable of firing bullets as other firearms, it was used as a diversionary weapon, capable of firing a variety of darts, similar to the blowpipes utilized by Assassins, and grenades, including poison and berserk ones.

Reference

 * YouTube: Assassin's Creed: Rogue River Valley Land Gameplay Walkthrough