Congrats on your shotgun, but it's still a shotgun, with all the limitations that come with it. It's still a devastating short range weapon, but ... well... look at the numbers.
The Remington Slugger ammo throws a 1 oz (437 grain) slug at something better than 1600 fps, for a little over 2900 ft-lbs at the muzzle. At 100 yards, that same 1 oz slug is down to a little over 1000 fps, with energy at about 1060 ft lbs. These figures come from an online
muzzle energy calculator.
Now, let's look at the common, standard .308Win. It shoots a 150 grain bullet at a little over 2800 fps, for 2649 ft-lbs. At 100 yards, it's still carrying over 2500 fps for 2137 ft-lbs.
Now, that's not to say that there is anything wrong with a 1 oz slug traveling 1600 fps. It's very effective, and plenty of folks use them every year to collect their venison. Police and military use slugs to good effect. A shotgun can be certainly used against soft targets within it's effective range. Remington Sluggers are good ammunition, and it is generally readily available. It'll certainly blow a hole in anything meaty, and it should penetrate soft targets (such as car doors) well. However, out past 100 yards, the slug loses energy quickly.
Still, it's a lot of fun to shoot slugs and buckshot through a shotgun. Enjoy yours, learn it, have fun. That's what this hobby is all about.