As in any internal combustion engine, there are a variety of chemical species present in the exhaust. Along with the variety of partial oxidation products from the methanol and nitromethane, there are also a variety of nitrogen oxides. Air is 80 percent nitrogen, and when you subject it to high pressures and temperatures, you get what are commonly referred to as NOx. NOx may combine with the water in the exhaust stream to form nitrous or nitric acids. Consequently, the exhaust from most internal combustion engines tends to be acidic and therefore somewhat corrosive. Thats why the exhaust pipe on your car looks the way it does after a few thousand miles. Put that acid in the air near your gun cabinets, and the results are what you'd expect.

Brad