Is the chamber for my shotgun designed to shoot 2 3/4, 3 inch shells, or both?

Normally the barrel will be clearly marked with the chamber length visible next to the gauge size.  A 2 3/4 mark means only 2 3/4" shells should be used. A 3 or Magnum mark means that 2 3/4" or 3" shells may be used. On some older shotguns you may have to remove the barrel and check for the stamped proof marks. Look for a marking CAM.70, CAM.76 or CAM.89. The "CAM" means chamber and the numbers are the chamber length in millimeters - the 70 mark for 2 3/4" chambers,  the 76 mark for 3" chambers and the 89  mark is for 3 1/2" chambers.