The government accuses my rifle an AR. They will have to prove it
Which they can easily do, simply by having a court recognized expert testify it is an AR variant. Then your defense would have to produce a recognized expert to testify it wasn't. And, somehow discredit the testimony of the govt provided expert. That's not an easy thing to do, and may not even be possible.
Simply put, if it looks like a duck, quacks like a duck, walks, flies, swims like a duck, eats duck food and breeds with ducks, its most likely to be ruled a duck, even if it doesn't say the word "duck" anywhere on it.