That is called a cannelure, and its purpose is to give a little shelf, on the inside of the case, for the base of the bullet to sit on. As you say, some have it and some don't. I ain't figgered out just WHY some use it and others don't. You find it in 38s, 357s and 45 Colts, also.
If you reload the brass, it irons out every time it gets fired, and long about firing #3 or maybe #4 it should be gone.
That cannelure is only useful for the original factory load and is removed by repeated reloadings and firing.
Today reloading dies for pistol cartridges work a little differently than reloading dies of old. The body of the case is sized a bit smaller than necessary. Then partial opened up (belled) for bullet seating. Once the bullet is seated there is a coke bottle effect below the base of the bullet in the finished cartridge. This all assures maximum neck tension on the bullet to hold it in place in the cartridge.
Pistol dies of old (at least the ones I used in the early 1960) left the bullet in the case only held by the crimp. You could actually spin the bullet in the case in some of them. This makes me wonder why anyone would buy old dies from a gun show (???). And those old dies, if old enough, where not carbide and required sizing lube during sizing.
Great info guy's. Thanks. I just got my hands on 30lbs of 44mag brass, Iam tumbling them and picking out the cases with out the cannelure for me. I just reloaded 1500 38 spl and could feel the cannelure being smoothed out on the first stage of my press.
case cannelure is to prevent bullets from being further seated by feeding in autoloaders and leveractions, as well as being under spring tension in a leveraction magazine under recoil force.
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