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SIGs are nice guns, but if you're going to keep it in your vehicle, you'll have to clean/lube it regularly to keep it from rusting. Especially in the Northern areas like MT and North Dakota where I live, having a weapon that is highly rust-resistant is a must if you plan on leaving it in a vehicle during cold weather. The hot/cold cycles from driving or just the difference between night and day temps can cause condensation on metal objects, and rust will be close behind unless it has daily care or high resistance to moisture.
I'd recommend a gun made primarily of stainless steel, adding a rust-resistant coating like NP3 to a weapon with normal steel parts, or getting a gun that is itself very rust-resistant, like a Glock.