I know there was a case where ammo (believe use of 10mm) came into question. I am sure I can track down examples where self-loaded cartridge used in SD resulted in legal troubles (that is a form of legal modifications). In addition, there is that case were a guy used some lighter strength ammo he had loaded for his wife, somehow that messed with the ballistics and made it appear that he was a greater distance. Lastly, a persons overall gun collection or group associations has been brought up (making the gun owner appear to be someone looking for a fight).
Now lastly, I believe there was someone with a a skull, or some remark, engraved on a handgun. This one I can't find, and may just be internet lies.
Now back to what pirate stated. Arguments of legal modified ammo is a simple step to legal modified firearm aside.
Are there any examples of a legaly modified firearm resulting in prosecution?
Are there any examples of a legaly modified handgun resulting in prosecution?