Where did that AK-47 come out of? What?
Void pouch. Thing they got from the prove-your-character challenge.
Priest guy that was training them brought him to the Isekai lost-and-found, filled with all the junk direct local transferees dropped on the ground, since no one knew which ones were useful and the void pouch's 'can only store any one thing a single time' restriction obviously wouldn't have proced for them even in a thousand years, given the items aren't native to this world.
Some camper got isekaied and left the bear mace, some soldier got isekaied and left his half-spent armament, etc. Up to you why the items were left though - could be they were instantly pressganged out of a ritual Reincarnated as a Slime-style, could be someone discovered their corpse following a mass-transference shonen-style initial character culling, whatever.