You can shoot the backdoor of the left car and open it that way. Now kill the zombies between the cars and the 2 zombies with critters in their chest that come out of the right car. After that the camera turns toward the left open car and you can shoot a special extra. There are more spots like that, but that should help you to get the point how to get those extras. Oh, all those extras are then only available in the NEXT game you start in Original mode.
Some extras can only used a single time, others seem to be saved and useable forever.
I have not found anything that tells you what differs the one from the others. You'll fire much quicker, it's like having a minigun. To Accesse the free play in the opions menu beat the Boss mode with all stars. Free play will be avalible in the continue settings. When you start a new Original mode game, the Goldman costume should appear in your trunk.