#nratt mod commands work. It can be used to make weapons fire once in two or three turns.
#selectweapon 22
#name "Balearic sling"
dmg -2
#dmg 10
#ammo 16
att -2
#att 0
#range 33
#sound 15
#nratt -3
#end
In other words, the negative number must be -(nr of turns). #nratt -1 doesn't do anything. #nratt -2 does. #nratt -3 does.
I found this out browsing through an ancient mod wishlist thread on the DomII forum after it was bumped.