Weird neck on 223 brass. Help me out!

[97 winchester]

That's a flash hole.

 

[Paxdad]

I still challenge the belief that this is from a heavy collet crimp as I have numerous crimps including those listed above an none come close to what the OP has in his picture. I have picked up spent brass just like his photo and resized/ reloaded it with no issue. I am going to go back to my original thinking and say that it is lands and grooves marks from a really tight bolt action rifle (the photo shows what would be 6 lands and 6 grooves). Again if anyone has a die that leaves this type of crimp I would like to know what it is.

 

[twinglock40z]

I still challenge the belief that this is from a heavy collet crimp as I have numerous crimps including those listed above an none come close to what the OP has in his picture. I have picked up spent brass just like his photo and resized/ reloaded it with no issue. I am going to go back to my original thinking and say that it is lands and grooves marks from a really tight bolt action rifle (the photo shows what would be 6 lands and 6 grooves). Again if anyone has a die that leaves this type of crimp I would like to know what it is.

I’ve researched it further and it is a crimp. It’s a crimp Winchester is putting on their military ammo. There’s also Winchester 308 with the same crimp. Ive also read on a couple other reloading forums that people have only ever seen this on the Winchester military brass with the head stamp of WMA.