Anybody know what this is,searched on google but can't find anything.I think it might be off a gun carriage?

Possibly a box that a soldier or officer kept his personal stuff in. Probably well before the 20th century but what do I know? It looks as though you can put a padlock on it which suggests it is not for shells. Have you googled regimental badges to see if you can find out which regiment/corps this might be? I'm assuming it is military!

I suspect the motto is Dieu et ma main droite – God and my right hand.

This is the royal coat of arms but used by a number of organisations for want of a better word. Some newspapers even had it on their banners. You can (could?) find it on a British passport as well.

Here's a link to a photo of the inside of the Lid,this why I think it is a shell box.
Photobucket

I think the motto is Dieu Et Mon Droit - God And My Right.

Also, 'Honi Soit Qui Mal Y Pense' around the centre.
'Evil Be to Him Who Evil Thinks', as in the Royal Coat of Arms.

Thanks for the replies,I've searched and found numerous images of the Royal coat of arms all with the motto Dieu Et Mon Droit literally God and My Right, meaning My Divine Right(to rule)But all differing in design.I was hoping to find the same design as the one on the box which might help to date it.It must have some age to it as the lid has quite a bit of wear, I presume, from being used as a seat.

Does look like it held shells judging by the indentations. The crown should give a clue as they changed with different monarchs I think.