At the pistol and rifle ranges I frequent nobody wears camo.
In the "Genuine Reason" section on gun licences and PTA's I can't recall "I like to dress up in military gear and a pistol/rifle will complete my outfit" being one.
While I don't think there are, or should be, any rules about wearing camo to a range, doing so says a whole lot of things about the wearer... and none of them are flattering, regardless of the size of the wearer's backside.