a crossbow powered by compressed air with a rapid fire clip, like in Van Hellsing

If a crossbow is powered by compressed air, it is no longer a crossbow, just a gun shaped like a crossbow.

Not neccisarily Hobo with a bazooka.

If compressed air is used in a sealed barrel to cause the arrow or bolt to take flight then it becomes a crossbow shaped bolt fireing airgun.

However if the compressed air is used in combination with a piston or pressure system to rapidly recompress the crossbow sprit and set the string it is an air compressed crossbow.

However using the system that retains itself as a true crossbow the rate seen in Van Hellsing would be impossible.


Remeber that a crossbow is a weapon that uses a mounted bow on a crosspeice with a pin retained string to fire a short arrow or bolt from a body.
What energysource is used to compess the bow and affix the string and pin is unimportant. Some crossbows used arm power, some dead weight, some winches.