Side note: the Perkins Steam Gun was a fully-automatic 19th century steam-powered cannon capable of firing over 16 rounds per second. It could send a steel pellet through 1/4" armor plate at 100 feet.
The two main power sources for the inhabitants of Refuge Base are high pressure steam and compressed air. These are used not only to drive a wide variety of tools and appliances, but also to defend the base and to arm their troops in Shadow. Gunpowder based weapons depend on the whims of a Shadow's rules regarding combustion and chemistry, while compressed gasses rely on much simpler physics.
Possibly the most intimidating and lethal examples of this would be the six dual-gun steam cannon turrets placed in and around the entry hall at Refuge Base. These twelve fully automatic steam cannons are connected directly to dedicated high pressure steam lines and manned by crews of artillerists and reloaders. Operating at full capacity, each cannon can fire a four pound shot every second. In several allied shadows where steam powered vessels are common, these cannons have become the standard close-quarters naval weapon.
For the military staff of Project TRUE, the standard sidearm is the semi-automatic pneumatic handgun. In many ways, these are simply scaled down versions of the steam cannons. However, where the steam cannons can be simply connected to the main lines, a handgun requires a much more portable source of pneumatic pressure. This drastically limits the number of times such a weapon can be fired, and alters the weapon's dynamics after each shot.
The handgun has dual reinforced pressure chambers, one in the grip, the other below the revolving ammunition cylinder. Both are connected to a single standard valve port at the rear of the weapon. This allows rapid re-pressurization from an ordinary steam or pneumatic line, but an optional hand-cranked pump may be necessary while in the field. The pistol's rotating ammunition cylinder holds six shots, usually the maximum that can be fired before the pressure chambers are depleted beyond useful capacity.
A pneumatic rifle has also been developed. It is capable of fully automatic fire at a rate of 10 rounds per second, and its longer barrel improves both the weapon's accuracy and muzzle velocity. In addiiton, the larger size of the weapon allows for more compressed air storage. A rifle can fire 50 shots without recharge. A backpack-mounted external pressure tank can boost the rifle's capacity by a factor of ten, and the weapon can also be connected directly to a pneumatic line, allowing it to be fired until the ammunition is exhausted.