extra cannelure on US A/P bullets

[RCS]

Some years I was given a few hundred 163gr US A/P bullets with the black tip, they had been
pulled from cartridges

My question, why do these bullets have a second cannelure ?


dscn2821.jpg

[Sunray]

That's a crimp groove not a cannelure. Most .30 AP was used in aircraft MG's so the crimping was necessary.

[RCS]

Sunray, it is the second ring or cannelure above the crimping groove - why the second cannelure ?

[Tuna]

To help hold the penetrator in place within the bullet when it hits a hard target.

[togor]

Some of the AAPs put them on in WW2, some did not. I have consistently seen those marks on Saint Louis and Frankford Arsenal sourced ammo. Those look like SL to me. The ammo I have from the other AAPs does not show the second cannelure.