Well C-Clamp grip is not something that's as straightforward as grabbing the rifle by the handguard, there are few mechanics too that work in our advantage.
As soon as a gun is fired it produces a "moment", rotate around a pivot near its chamber with the muzzle going upwards; the muzzle flip.
Using the simple formula of torque, we know that lever arm X force is torque. So as you're increasing the distance of C-clamp from the chamber, the torque you'll be able to apply to counter the torque applied by the gun during muzzle flip is also increasing. In simpler terms, the farther your hand is the lesser muzzle flip you'd feel.
Also wrapping the grip around the barrel makes it slightly more precise to adjust than a fore-grip though this is something too minute for a regular Grunt to realise.
This is why you'd see shooter placing their hands on the handguard as far away as possible
But if you try to replicate this thing on a bullpup then your hand would be right on the muzzle attachment...which I don't think would be a pleasant situation