'A bunch of car batteries' does not equal peak-discharge batteries -- which are batteries with a capacitor that charges up to a level much higher than the constant charge that the batteries have, then releases it. With a gauss gun, it releases it in sequence to the electromagnetic rings, yanking the ammunition forward -- but the amount released at once is much higher than the battery can do on its own.
Think of it as a water source. The capacitor is the 5-gallon bucket that you're filling up from your kitchen sink (the regular battery). It takes a bit, but you get to the top eventually. Now, instead of the kitchen's moderate but steady stream, you could go to the outside hose (your Hick Science) and kick THAT on. Bucket fills much faster, doesn't it? However, even the outside hose's stream is not as much water in one moment than the 5-gallon getting instantly dumped on you.