Snuff introduces you to both a new character in JD and also the weapon shop Ammunation.
At this stage of the game only the handgun is available, but more weapons will be sold there as you progress. And of course you can pick up just about any weapon for free if you know where to look (see our other posts in the Walkthrough link above).
When you reach the construction site, select your weapon and take out the guards. Walk through the entrance they were guarding and keep firing picking up the free ammo along the way.
This mission also introduces you to the Pay’N’Spray concept where for a mere $100 you can repair and respray your vehicle, losing any wanted stars in the process, unless you try to use it when in a police vehicle.