Dual valves

[frankrede]

The third gun would have slightly more power due to teh added chamber volume.
But with your insistence on shit valves it wouldn't make a difference.

but the first and third would perform best.
The first would be cheaper

[SpudMonster]

You will want a minimum amount of dead space between the valve and the projectile's ass end. Putting the valves in-line with the barrel would be a major improvement.