Single-shot rifles, bolt-action rifles, lever-action rifles, and semi-automatic rifles (non-NFA)...
Single-shot shotguns, double-barreled shotguns, pump-action shotguns, or semi-automatic shotguns (non-NFA)...
Single-shot pistols, semi-automatic pistols, or revolvers (non-NFA)...
All of these items require only two things in the grand state of Missouri: a properly completed Form 4473 and a "pass" of the NICS/FBI background check that's called in after 4473 completion.