- The weapon will have short range if the net is deployed right out of the barrel, so you would need some kind of unfolding mechanism (advanced technology or magic) to hit something more than a few yards away.
- The net need not to be ruined by the act of firing (either use catapult/ballista instead of cannon, invent an impact and fireproof net, or use magic).
- You need to hit your target (to hit roll or magic).
- Once unfolded the net needs to envelop the target (skill check to see if you timed it right or magic).
- Once enveloped, the net has to lock to the ground (definitely magic).
- I'm assuming they want to catch soemthing scary with this net, so it needs to be strong enough to hold it (advanced technology or magic).
I think the queston is if your players aren't trying to stomp ants with orbital warheads.
The kind of investment of time and energy needed to create such a device could probably be used more economically if their objective is simply to disable someone.