Guns in trunk of car?

Picking them up off of dead enemies tends to work as well.

I'm pretty sure you can only scroll through the weapons if you get the DLC bonus Chicago Piano Gun which you can use from the trunk.

As far as I know there is only one mission where you can pull guns out of the trunk and you pull shotguns out. The Revolver is wielded during patrol. Handgun is the normal for the rest of the game. Special Weapons are obtained by picking them off dead enemies. Shotguns and Thompsons are the usual drop. Very rare occasion you can use the BAR, M1 Garand, and the flamethrower. Chicago Piano Gun is DLC.

But Shotguns are pulled from the trunk only and no there isn't a scroll.