Are these airguns full metal?
Alkali metals and alkaline earth metals are all too reactive to be practically made into coins. Ideal coin metals are fairly inert, you can't use something as currency when it bursts into flames the moment you take it out of your pocket. This is why noble metals such as gold and silver were so popular as coin metals in the ancient days. However due to the rarity of noble metals and the amount of trade in the world today, base metals are used as coin metals instead.
Group 1 and Group 2 metals are very reactive, i.e. sodium and water. Transition metals can be much more chemically stable.