shell size for 410 shotgun

Question
I have inherited an old 410 double-barrel shotgun.  I do not know which size of shell to use:  2 1/2 inch or 3 inch.  Brand name is Crescent Davis, ser #2239.  The guy at the ammo counter said that it should be printed on the barrel of the gun, but is not.  I would like to clean the gun up and practice, but do not want to damage using the wrong size shell.  Can you advise?
Thank you.
Jim

Answer
Jim,

The difference in those chamberings is in the size of the forcing come. A gun chambered in 2 1/2 has a shorter forcing come, and will not allow the shell to open all the way, which drastically (perhaps catastrophically) increasing pressures.

Since 2 1/2 was the original chambering and 3 inch came later, if the gun is not marked, it probably will only handle 2 1/2 shells. The thing to remember with shotguns is that the gun can handle shells shorter than those specified but not longer, so even if it does have a 3" forcing cones, 2 1/2 will be just fine in it, but not the other way around.