barrel length

Ok I am looking to get into Hunting, and I was looking at Turkey season. I have a Mossberg 590 Retrograde 18.5" Barrel pump action shotgun, and I was wondering if this would be ok to hunt with? One of my co-workers said that the barrel is too short and to get an actual Turkey Shotgun. However...

When we measure the barrels on shotguns, rifles or revolvers the barrel begins where the chamber ends. Pistol barrels' spec sheets include the chamber length as the 'barrel.' Being a stickler for logic it makes no sense to me to change the method of measure for pistols. It can be misleading...