Best to keep it as stock as possible. There's nothing wrong with it, and the laws are so confusing, nobody will be able to correctly tell you if it's legal or not. Some of the "semi-detachable" magazines are unreliable and are all slow to reload. Stick with 10 the stock magazine and stripper clips. Best to have a way of locking it up if you're going to leave it in the car, too. And don't forget to keep it oiled for corrosion resistance. Here in FL, long guns in cars are mostly regulated by laws relating to hunting, while the CCW laws concern mostly handguns. I am not a lawyer, thank goodness.
And if you have the Yugo SKS with the grenade launcher, YOU ARE MISSING OUT IF YOU DON"T OWN THIS:
http://www.e-gunparts.com/DisplayAd....rSuperSKU=&MC=
Available as a kit or as individual rounds.