To continue with the pedantry of the comments, it really depends on how you define the ownership. It may have been the Spanish who owned it once upon a time, ho

To continue with the pedantry of the comments, it really depends on how you define the ownership. It may have been the Spanish who owned it once upon a time, however they left it on US soil. Additionally, it was Americans who found it, and if we assume “finders keepers losers weepers” applies then the US is, in fact, the rightful owners, and unless actions are taken to retrieve it then it will go to an Arizona museum, solidifying the ownership as American. If we wished to be technically correct (which is the best kind), then we would say a “Spanish-used antique firearm, which is not legally a firearm, unearthed in Arizona, now United States’ oldest firearm.”