If we could viably ensure/trust that the buyer would use the firearm/gun for something other than bringing harm to another human being then we would not restrict this type of item, however it would be impossible for us to do so, hence the restriction. Its not an issue with the gun/firearm itself.
No drugs then either!
They are not restricted, chemical weapons are naturally restricted.
If someone wishes to purchase and use a substance that will alter their own body or mind, that is the decision of the buyer.
If people wish to buy weapons there are other platforms available to purchase them with Bitcoin online.