The only Runtbot rules I know of being out there are incredibly outdated from the 2nd Edition book 'Ere We Go' where the Runtbot was controlled by a Runtherd that was at a maximum of 16" away (the smart opponents would presumably kill the Runtherd to disable the Robot).
It had a real wonky Kustom Force Field that functioned a little differently then today's (units within 6" were protected and couldn't fire out of the field) as well as a Smoke Stickbomb launcha that would fire 2 of these 3D6 inches in a random direction (note random and not scatter).
The Ere We Go book is rare and hard to find, so if you want the rules from there you can PM me, although they are extremely dated at nearly twenty years old and I'd hope that the Runtbot would have other rules out there somewhere.
Peace
~OD
Post Script: Although it doesn't exist yet, it is likely that Forgeworld will publish rules for the Runtbot (as well as many other Snakebite-esque stuff) in the next Imperial Armor, as they've already said that the next Imperial Armor will cover rules and background for Bad Moons, Snakebites, and Blood Axes.