There are also the Artificer class infusions (effectively spells) that grant temporary weapon, armor, and shield properties to items, meaning Metalline can be briefly added to weapons via 'spells.'
Where are they from Endarire? Well thanks for the feedback, I think you're the first person who has said they used them. The stacking thing sounds like its definitely a bug, and yes, like most alchemical items it is for low level characters - something to have in case you run into a monster with DR before you get the appropriate items. They were originally put in KotB to be made at low level with the Alchemy skill and were then added to Verbobonc because, why not? (The only critter with DR in KotB that comes to mind, other than skels and zombies, is the Gargoyle in the Mad Wizards tower. Anyway, the positoxin one, that coats your weapon with Holy Water, is an interesting one to have).
The 3.5 Artificer is from Eberron Campaign Setting, a class focused on crafting stuff and being natively proficient with repeating crossbows for some reason. (Maybe they're 3.5's closest analog to guns.)