From the MDC's website page HERE: http://mdc.mo.gov/hunting-trapping/regulations/deer-regulations/2012-firearms-deer-hunting
The part that really matters to this season is this:
Alternative Methods portionTo offer hunters more options during what was formerly the muzzleloader portion of the firearms deer season, additional hunting methods will be allowed in 2012.
- Dec. 15–25 statewide
- Muzzleloading or cap-and-ball firearms .40 caliber or larger and capable of firing only a single projectile at one discharge; in-lines and scopes are allowed.
- Multiple-barreled muzzleloading or cap-and-ball firearms and/or muzzleloading or cap-and-ball handguns, including revolvers, .40 caliber or larger, are allowed and may be carried in addition to a muzzleloading or cap-and-ball rifle.
- Centerfire pistols or revolvers using expanding-type bullets; legal ammunition includes lead bullets, copper bullets and bullets made of other material designed to expand.
- Air-powered guns, .40 caliber or larger, charged only from an external high compression power source (external hand pump, air tank or air compressor)
- Longbows, compound bows or recurve bows of any draw weight; hand-held string-releasing devices, illuminated sights, scopes and quickpoint sights are allowed.
- Atlatls, which are defined as a rod or narrow board-like device used to launch, through a throwing motion of the arm, a dart 5 to 8 feet long
Some of us magnum and big bore revolver shooters use hard cast lead bullets that don't expand - a .44 or .45 caliber flat-nosed slug doesn't really need to - so I contacted the MDC about that part of the regulations and they said as long "as it's lead it's legal". So there you go.
Strap on you revolver, air rifle, or atl atl and go hunting.