Just yesterday this video of Mike Shake popped in my feed: https://youtu.be/tHzuz09l41U

Essentially a sniper rifle with compressed air able to propel a 3d-printed bullet at sub-sonic speeds able to break a simulated skull. And it penetrated hard enough to for sure cause major complication for the target. Not to mention that lead slugs can easily be made DIY without much complications and skill. Lead can be melted pretty easily so lol.

Not much to do besides being able to cut a pipe, make a release mechanism and compile it all with a projectile to shoot. Done is your makeshift weapon.