The safest way to make sure that you can't be in trouble as a felon in possession of a firearm is to keep them out of your house and car. Anything else is not 100% safe. If you live alone, safe or no safe, there should be no gun in your house.
If you've got a spouse or adult child who still has gun rights, if it's in a gun safe, and you don't have easy access to its location within the house and you don't know the combination, then it should be okay. BUT, a lot will depend on the circumstances under which the gun is discovered by law enforcement. You can bet that if there's a domestic violence
call to the home, for example, the gun will be confiscated and if it's determined that you could have had access to it, you could be charged.
A felon may not knowingly possess a firearm. See link.
The state of Missouri defines a firearm in the beginning of chapter 571
of Mo Revised Stats as any "weapon that is designed or adapted to expel a projectile by the action of an explosive
" So if it uses an explosive to eject and propel the pellets it would be a firearm. If its an air gun, it is not a firearm and would be lawful for a felon to possess and use in Missouri.