How heavy would an over-under break-action grenade launcher, enlarged to fire 40mm grenades.

I'm writing a war novel set in the near future, where the characters use unorthodox weapons with heavy firepower during firefights.

My story's hero invents what's basically a super-sized over-under, break-action shotgun, a grenade launcher that holds two 40x51mm grenades, one above the other.

I chose this unusual weapon for my hero because he needed something that can blast through body armour designed to stop rifle rounds, yet light enough for him to carry around for prolonged durations.

Modern body armour can stop up to 30-06 Springfield. A 338 Lapua Magnum rifle can punch through that, but it's much too heavy for an infantry weapon.

In the end, I decided to equip my main character with a 40x51mm grenade launcher, but multi-shot designs can be quite heavy. So, I decided to go as simple as possible.

A 40x51mm grenade launcher based on an over-under break-action shotgun can, theoretically, be reloaded quickly and the relative lack of moving parts should keep the weight down. Most importantly, a 40x51mm grenade can easily blast through body armour from 400 to 800 meters away.