Yeah, its like a .380 ACP and 9mm. Similar dimensions, except length, but you can't shoot one from a gun designed for the other.
ulm, the 9mm case has a slight taper to it whereas the .380 is a straight case.
Also the .380 case is designed to have pressure blowback around the case to cycle the action, the 9mm uses recoil.
You can fire a .380ACP round in a 9mm gun for sure, but you will notice alot of blowback (black scorch marks) all around the case of the .380 if fired in a 9mm chamber. It is theorized that continued shooting of a .380 in a gun with a 9mm chamber will eventually damage the metal in the chamber because the gas isn't properly sealed with the .380 case. The gas is hot enough to "cut" the metal from what I understand. This would especially be bad if the frame is either aluminum or polymer. You would also have to cycle the action on the gun everytime you wish to shoot.
.380 in a 9mm not a good idea and never should be considered unless in dire of circumstances. Ofcourse, NEVER try to fire a 9mm round out of a .380ACP pistol. Even if you manage to get the action to close, the pistol would blow up if a 9mm round was fired in a .380ACP pistol.