Actually they came out in 1975 or so. The 6BR works better for heavier bullets and longer range. I haven't seen the 6BR used at all in short range benchrest in years. Necked up to 30 cal the 30BR with 112-118 grain bullets dominate the short range score shooting.
Unless you are shooting UBR the 30 has a distinct advantage in score shooting which is why not many shoot the 6 anymore.
