I know some of them were modified to fire full auto
That and being as it isn't registered through the BATF as a shooting rifle, then it is illegal if you do anything to it. And if it does in fact have a suppressor at the end, then again illegal as hell. I wouldn't even want to have pictures of it, or be associated with it.
Now you can have it registered through the BATF but it will take time and money to do, and to be honest...... it wont be worth it. Keep it as a wall hanger and leave it at that.