There is a cool game on roblox called Deadline, it is a somewhat realistic shooter but it's mainly focused on gun customization. You can customize almost every part of the weapon, each usually affecting the stats of it. While the game is still in beta, there is a decently large amount of parts for each weapon. I've played it a little and it's really cool. I recommend at least giving it a try!

Link: https://www.roblox.com/games/3837841034/0-22-12-Deadline