I have one of these, too. They're surprisingly fun little guns.
About the build material, like many of Marushin's other guns (as well has WA) it actually isn't full metal but Heavy Weight (HW) ABS, which contains a mix of metallic powder. It does feel quite convincing, however, as you witnessed first hand
Second, as is typical of Marushin engineering, the reservoir and firing valve are all built as one big "gas block" which fits inside the grip. Unfortunately in the case of the Derringer, since the sides of the gas block make up the grip panels, one side is removeable; the reservoir is kept sealed by a rubber member under the removable panel. This is far from optimal, unfortunately, and using anything stronger than duster will quickly cause a leak. I tested mine with green gas/propane, and there was little by way of increased performance anyway -- the barrel is just too short, even for the heavy weight 8mm BB.