The 243 came out in 1955. It's been a standard since about the time it hit the shelves. The 244/6mm came out the same year but the twist was too slow for heavier 6mm bullets. The 244 is 1-12 twist and the 6mm is 1-9. Funny thing is Paul Mauser necked down the 7mm Mauser to 6mm in 1899....which is for all intents and purposes what the 6mm/244 Remington is. 7mm Mauser neced to 25 cal. for the 257 Roberts and they took the Roberts case and necked it to 6mm. Shoulder angles might be a bit different but, they're both based off the 7 X 57.