YouTube: Video Making A Dummy Round
Because Greg builds chambers the way you prep your brass and the depth you seat your chosen bullet, he requires a dummy round. If you have more than one bullet, he needs a dummy round for each. He then chambers to the longest throat length. If you are going to shoot factory ammo, he needs the rounds to build around the factory ammo. Keep in mind that whatever brass you send, that is what he will build your chamber around. If Greg has any concerns or feels there are any mistakes he will call you.