You cannot patent an idea; IIRC, you don't need a working model any more, but the "thing" has to be more than a vague drawing or idea, often totally impractical. Most of the ideas I see on here are in the latter category, usually prefaced by something like "I have this great idea for a handgun chambered for a 16" naval shell..."
Just from the few words you gave, I can see several problems, not the least of which will be the legality involved in actually producing anything that big. Then there is the guidance method. You need to get the electronic and mechanical steering into a fairly small projectile, plus the means of knowing where it is going, so it can be steered (a radar or small TV camera of some kind?). And of course it has to be fast enough not to be shot down, yet slow enough that it can be directed in some way (laser aiming point projected on the target? More "stuff" to put into that 40 mm).
Jim