This has been beat to death for years. The neck on almost all 223 is too long and will not close on 5.56 ammo, almost all!
5.56 brass cut to length is just fine if you load lite and work up.
Don’t believe me? Go buy some HSM 223 reman ammo, they sell it commercially as 223. every. single. head-stamp. 5.56.
And HSM ammo lived happily ever after. (Stevensville Montana, Right by Cooper Firearms, Just south of Los Angeles de Montana)
THE END.
