No, in your case the crime isn’t the fact that you modified the rifle, the crime is that you modified it into an illegal version of the rifle. The crime is possession of a full auto rifle.
If I take a rifle that was setup for one caliber and modify it so it can shoot a different caliber, that’s not illegal.
It’s not illegal to modify a gun, it’s illegal to modify a gun into a gun that meets certain criteria to then become illegal. The crime isn’t modification of the gun, the crime is the possession of a (now) illegal gun.