The 7mm Rem Mag was introduced in 1962 and first used in long range matches in 1970. That's when the first 28 caliber match bullets were available. A friend borrowed the scoped rifle and ammo from another mutual friend to break the 1935 record shot with a 300 H&H Magnum at 1000 yards.
https://sierrabullets.wordpress.com/...wimbledon-cup/
https://www.ssusa.org/articles/2020/...rsial-sighters