Small caliber guns (like a 9mm Glock for instance) work if you manage to aim correctly at the brain stem in the mouth and slightly up, account for recoil and use appropriate ammunition (hollow point in that case). You should have fired the gun several times or optimally practiced at a shooting range though, doing without prior experience is a huge gamble as the recoil can make you miss entirely.
Shotguns are the recommended way and might be easier to obtain. The firearms megathread is quite helpful here. Good luck!
https://sanctioned-suicide.net/threads/guns-firearms-megathread.9042/