Sorry, but you guys have been given some bad information.
First, the 223/5.56 is a high pressure round and the Rem 7 1/2 primer is the correct Remington primer for high pressure rounds. The Rem 6 1/2 primer has a very thin cup and is designed for low pressure rounds like the 22 hornet.
From Remington's web site.
In rifle cartridges, the 6-1/2 small rifle primer should not be used in the 17 Remington, 222 Remington, 204 Ruger or the 223 Remington. The 7-1/2 BR is the proper small rifle primer for these rounds.
BTW Rem 7 1/2 primer are all I use in 223/5.56, 6.5 Grendel and 221 Fireball luv em great primer.