Steam powered guns
The problem with steam guns is that they're close-combat weapons. Steam
traps would be awesomely dwarfy and ridiculously badass (not to mention efficient- scald shit to death without requiring drainage and you even get to keep all their gear), given their high potential for indiscriminate killage, but putting a steam gun on a mech or using one for forward defense isn't cost-effective, unless you have so many guns or so many siegers that you can mitigate the very very low chance of actually hitting anything with a particular gun.
and transportation.
Which is sort of what I meant by a power pack for my mech... it'd be a little backpack-type construction with a water-wheel-turned-steam-turbine, a steel bin with some burning lignite and a condenser. The wheel/turbine would power the axles that move the arms and legs. And it'd have ballistae on its shoulders and I would call it the Berzerking Stray Cat.
And stroke its fur the wrong way and call it George.Just imagine it. I fill a cannon with my millions of stone crafts, gobbo bones, and some barrels of booze. Then build up the pressure and release it all at once! Fiery, stabby, boily, crunchy death.
...You heathen. Unless you have a rotating crew of peasant brewers making no-quality gutter cruor, that's a criminal waste of alcohol.
On the other hand, shrapnel cannons... do want. Hell, even improvised catapult ammunition would be awesome. (Hurling burning graphite, aw yeah.)
Or... wait. Supposing you had a dorf put something flammable in a bin atop a hatch. Then a lever is pulled and the bin falls down two z-levels, first past a magma waterfall that generates mist that sets the contents on fire, then into the chamber of a slingshot-type weapon...
Also, Do want battlemech.
Promise I'll get right on that as soon as my railgun's done.