The other marksmen shooting in those 'Big Bore Leagues' used the rifles of the era which included M70 match rifles in 30-06, Springfields and a few customs. I thought the 243 was an advantage as it has a lot less recoil.
After doing well with it I ordered a Rem. 40X match rifle in their 6mm Rem International chamber which is a 250 Savage case necked to 6mm. I shot the 75 gr Sierra HP over 30.8 grs of 3031.