[Takeda Shingen]

Ya'll like Wikipedia so much that I went out and quoted it for you:

A weapon is a tool used to injure, incapacitate, or kill an adversary.[1][2] Weapons may be used to attack and defend, and consequently also to threaten or protect. Metaphorically, anything used to damage (even psychologically) can be referred to as a weapon. A weapon can be as simple as a club or as complex as an intercontinental ballistic missile.

An airgun seems to fit the definition of weapon. Webster's seems to agree as well. I don't think that I see any bias.