Splint cannons on moving vehicle? fire heavy or assault??

this rule "that vehicles fire the heavy option in a gun" eg. venoms fire heavy X blah blah...

soo im just wondering, if a unit in a vehicle, moves and has the splint cannons or any other gun that has the option to switch? do u fire it as the heavy option as well??

Models firing from a transport count as moving if the transport have moved. So warriors with splinter cannons in a moving transport use the assault value for the cannon.
And remember, if you move at Cruising speed, or faster, they may not fire at all.

sweet thanks!

If you move your transport at cruising speed, the unit can disembark and then fire though, correct?

Right, unless you move flat-out, you can disembark and simply count as moved.

Also a question, is does 18" count as flat out? And 17" is the highest cruising speed u can get and still be able to get out from?? (hopefully that made sense)

Stationary is 0", up to 6" is combat speed, up to 12" cruising speed and everything over 12" is flat out.
It is unimportant, how you move, for example with sails or not. If you move over 12"* you can't dissembark.

Only exceptions are red orks and wheeled vehicles on roads.

But how bout our open topped fast vehicles??

Makes no difference, move more than 12 and your staying aboard.

Oh ok! Cool! Thanks!

open topped fast vehicles just lets you assault after disembarking