Here are some pictures of how I am using the extra gun barrels from O8´s M92 SPG to build howitzers for my 3mm Ogre set-up.
Here are the supplies and components:
A bit of pasteboard for the base. A thumbtack (pointy bit cut off and reused as a bunker antenna), eight slices of a dead credit card, a couple of plastic bits (which are glued together and filed into the right shape), some plastic tubing (for the bunker) and, of course, the extra gun barrel from the M-92.
This shows you the four howitzers in various stages of construction.