To answer the title, yes there are open source gun designs. The FGC-9 comes to mind. This is NOT an endorsement to build any of this, I just wateched a Vice documentary on this recently.
Build responsibly, check your local laws regarding manufacture of unserialized firearms and use of force. Learn safety before you shoot yourself in the foot, literally, or shoot someone else. Keep it out of reach of children.
One thing I really liked about tacticool girlfriend's YouTube introduction to guns was that early on it goes over first aid for accidents that can happen and safety to ensure they don't.
And absolutely, first aid is essential. If you're gonna play with things that can injure or kill things in the blink of an eye, you ought to learn how to handle that. I sometimes joke that the kink community puts so much emphasis on consent, negotiating kink interactions, aftercare, etc, because if you're gonna choose to play with fire you ought to learn how to use a fire extinguisher.