Thanks for the input. I have been thinking about your comment: "Right now it look like it would go rolling backwards one it starts shooting." I don't want to add any support legs or anchors, I'll just leave at is. I could say that the wheels have brakes and leave it at that, but that is no fun. I've thought about coming up with a special rule for the recoil, a variation of the Don't Press 'Dat rule.
Here is what I'm thinking: Before firing, roll a
D6, if a 1 is rolled than the brakes have failed and gun travels backwards 6 inches due to recoil. furthermore, due to the grot inside being thrown of his aim, the ballistic skill is reduced one to Ballistic 2 (normal ork ballistic skill). Anyway, this is a start I'm sure that this rule could be improved. In the long it may just be easier to add support legs or anchors, but randomness is what can make orks fun to play (and at the same time, aggravating to play, especially when on turn one your Big Mek with
SAG kills himself or teleports into base to base with a squad of terminators

)