Either make it a fire and forget drop weapon (in the way of the Pipebomb), or make it a hard hitter weapon (in the way of the LAW).
1) Buy the gun, minimal weight, like suggested somewhere above. 2 Loads, costing 400 each (except for demo, usual cost bonus; demo can carry +1 per level). Explosion damage like pipebomb, but smaller radius since it can be aimed and triggering remote bombs can be more difficult / damaging to self.
or
2) Make the LAW a force. All bonusses apply for Demoperk. Weighs 6-8 pounds (I need to check to balance out with M32), costs around 1200 with only 5 shots + ammo (10 per shot). Max ammo is 30, lower than Xbow, without piercing values, but weighs less and has blast radius. It's also slower than the xbow and produces smoke. Makes it an extremely slow, hardhitting weapon for demoperk.
€ Oh right I forgot, damage needs to be adjusted (heightened) in order to balance it out. Kinda like 2000 seems right with perk bonusses calculated in. Of course this would need to be checked ingame.