Guns for the Martians: Air Rifle

As endless strife turned to lasting peace, the Empire set out to outfit the armies with new weapons. The more civilized age demanded an elegant weapon, safer for the soldier and which could make him more independent of supply chains. The actual origins of the air rifle's design are lost to time: some claim an epiphany from the Wind Genies, others, that they were forged after the evil Old Martians' bee rifles.

That peaceful age fell woefully short of expectations, however. After hurriedly reverting to the older fashion and especially after the Fall, these guns were appreciated by peasants and petty nobility for small-game hunting, as their shots wouldn’t make the whole game population run or fly away. Abandonment, poor maintenance and lost knowledge on how to operate them made many of these weapons fade away with their last owners.

 

What's this? (TLDR) A moderately-damaging gun with a usage die mechanic that doesn't however expend precious resources (ammunition). In other words, a +1 bow that runs out of juice after a while.

Girardoni air rifle. See it in action here

A conventional air rifle can take a single shot each round. As a gun, it gets +1 both to hit and damage for every shot each round including the first (as per GLOG gun rules), which in this case just translates to shooting always with that +1 bonus.

It takes up 2 inventory slots, plus 1 extra slot for the weapon’s tool pouch (cleaning stick, hand pump, bullet mold and spare ammo balls). It shoots minute metal balls, trivially made from scrap metal with a mold and a fire. As such, ammo isn't tracked as long as the weapon’s pouch is carried (always assumed to have enough balls, just like arrows in a quiver). Likewise, the rifle's ammo tube holds enough balls so as to effectively ignore reloading, tracking instead the air pressure in the reservoir.

Kunitomo air gun

Note the animal-like (fox?) hammer

An air rifle's damage die acts as an usage die, representing the diminishing air pressure in the reservoir, as follows:

d6 > d4 > d2 > empty

When a 1 is rolled as damage, the die steps down a size. On stepping down from d2, the pressure becomes too low to inflict any significant damage and the weapon must be hand-pumped again. The process is long and laborious, taking up an exploration turn for each step.

Such a weapon has multiple advantages over ordinary firearms, namely being much quieter (while not totally silent, stealthy enough that the shooter’s position remains concealed and doesn’t trigger an automatic encounter check when firing) as well as firing without smoke nor muzzle flash.

Along with an air rifle, an Imperial foot soldier would be equipped with this mass-produced armor:

Korean fabric armour (Source).
As Middenmurk pointed out, looks like a potato sack

 
These single-shot rifles were, however, for rank-and-file conscripts. More advanced air rifles with a hammerless mechanism were devised, allowing for up to three shots per round at the cost of expanding the step-down range on the damage die for each additional shot (so, for example, taking two shots for a +2 to hit and damage at the cost of stepping down the damage die if 1-2 are rolled) and taking up an extra inventory slot. The few surviving ones are now true relics.  

 

Reskinning this for your own game: a rules-light version of the air rifle could be using it simply as a reskin for the regular/light (whatever you call "non-heavy" if there is such a divide in your game) crossbow: d6 damage, shoots once per round at +1 to hit and damage. 

Similarly, for more science-fiction-oriented games, the air rifle could be styled/renamed as a wind-up gun, a crank laser musket, or whatever.

Air gun with hand pump. Source