Some method of portable target, shooting range or the ability to make a shooting range of sorts from things that already exist (such as a six-pack of cans or bottles). Could be something marketed, could be something crafted, could be just something you could do with a bottle placed in a room or something.
This wouldn't calculate points or anything, but might be a fun way like a dart board to test skill with a firearm, and would help those who are pursuing firearms illegally still RP practicing and learning the skill in a manner that isn't just handwaving.
Let me know what you think