I looked at my primer supply. I had a wooden box of primers I've had since 70's, the 7 1/2 boxes say bench rest.
As I recall the 222 Remington used 6 1/2 primers way back when. I bought a 1,000 of the 6 1/2's last summer on the way back from PD shooting. I promptly lost those primers. I had planned on using them in 17 Hornet, 256 and 222.
I believe the 7 1/2's were brought out for the higher pressure than the 222. The magnum moniker may perhaps have referred to usage in the 222 Magnum? That is my guess.
