Another vote for an SKS. I have one I keep in a cheap long gun sheath that zips up behind the seat. It's kept unloaded, and I have 4 stripper clips with 10 rounds of soft points each in the glove box.
The rifle is beat all to hell from pulling years of duty as a tractor gun, but it shoots just fine. I just spray painted the dinged up wooden stock with flat black paint and theirs your " black rifle".
You can pick them up for around 300 bucks.