All I know is I have a 223 WSSM and I do not own a 243. I'd like to let my kids use the 223 for deer, because there's virtually no recoil, and a 65 grain bullet at 3,400 feet per second will probably kill just as well as a 243 Winchester. In Wyoming it's legal, and other states as well.
Just did a search and came up with this list, it's not up to date so some states might not be correct. I updated Wyoming and Kansas.
Alabama- centerfire rifle or pistol using mushrooming ammo
Alaska-
http://www.adfg.alaska.gov/index.cfm?adfg=hunting.firearmsAlberta- .23 and up centerfire
Arizona- centerfire
Arkansas-
http://www.agfc.com/hunting/pages/huntingregulationsdeer.aspx#Legal Hunting Equipment
California- centerfire rifles, centerfire handguns with min. 4" barrel
Colorado- .24 and up, 70grn or larger bullet/ minimum of 1000ft/lbs at 100 yards
Connecticut- .243 and up if legal in your area
Delaware- shotgun/muzzle loader
Florida- centerfire
Georgia- .22 and up centerfire
Hawaii- Any rifle with at least 1200 ft/lbs of ME. This would start at around .223 I think
Idaho- Centerfire (cannot weigh more than 16 lbs?)
Illinois- Shotgun/ML/Pistol only
Indiana-
http://www.eregulations.com/indiana/hunting/deer-regulations/Iowa- .24 or larger centerfire only for antlerless season in part of the state.
Kansas-
http://www.kdwpt.state.ks.us/news/Hunting/Hunting-Regulations/Deer/Legal-EquipmentLouisiana- .22 and up centerfire
Maine- .22 magnum rimfire and up!
Manitoba- Centerfire, but it says .23 and below not recommended. Does not say illegal though.
Maryland- ME of at least 1200 ft/lbs
Mass- Shotgun/ML
Michigan- centerfire in certain areas
Minnesota- .220 and up centerfire
Mississippi- No restrictions that I could find
Missouri- centerfire
Montana- No restrictions
Nebraska- Rifles with 900 ft/lbs or more at 100 yards
Nevada- .22 centerfire and up
New Hampshire- centerfire
New Jersey- shotgun or muzzleloader
New Mexico- centerfire
New York- centerfire
North Carolina- No restrictions
North Dakota- .22-.49 centerfire
Nova Scotia- .23 and up
Ohio- Shotgun/ML
Oklahoma- centerfire with 55 grn or heavier bullet
Ontario- centerfire
Oregon- .22 centerfire and up
Pennsylvania- centerfire
Quebec- 6mm/.243 and up
Rhode Island- shotgun/ML
Saskatchewan- .24 and up
South Carolina- centerfire
South Dakota- rifles with 1,000 ft/lbs or more ME
Tennessee- centerfire
Texas- centerfire
Utah- centerfire
vermont- No restriction
Virginia- .23 centerfire and up
Washington- .24 centerfire and up
West Virginia- .25 rimfire and up and all centerfire
Wisconsin- .22 centerfire and up
Wyoming- Any center-fire firearm of at least .22 caliber (excluding .22 Hornet) and having a bullet weight of at least sixty (60) grains and firing a cartridge of at least two (2) inches in overall length