WHAT are these crimps?

[4godsako]

I've just bought two boxes of ammo for my Rossi/Puma .38/.357mag and I don't understand why the cases have crimps in them?
The first type: .38special 110gr JHP(+P) have a crimp 8mm down from the case mouth and it's nowhere near thebullets base so it's nothing to do with holding the projectile , as I wrongly thought it was intended for???????(Winchester brand)
The second round is a Winchester .357mag 125gr JHP and this nickle plated case has TWO crimps in the case's circumference...one 10mm down from case mouth and another 2nd one 15mm down from the case mouth.
Can anyone tell me what these rings intended use is and why when I see other .38's---.357's cases , they don't have them.
Taa. :banghead:

 

[Freedom in theSkies]

In my experience, the crimps you are reffering to are there to prevent the bullet from seating deeper into the case, when the gun is in recoil. Probably more common on 9x19.
hope this helps.

 

[stans]

What you are seeing is a cannelure on the case. This is done to prevent the bullet from being pushed deeper into the case. Revolver rounds are also roll crimped at the case mouth to prevent the bullet from being pulled out under recoil. Semi-auto round are taper crimped at the case mouth. The practice of case canneluring varies based on ammo manufacturer. Winchester loves them, it was common to see two cannelures on their 38 Special cases. The practice seems to be losing popularity. I have noticed that Remington used to put a heavy cannelure on their 9mm ammo, the last three boxes of +P I bought are not cannelured. I think manufacturers are relying more on the sealant they apply to the bullet to prevent movement.