With my large collection I picked from the best shooters and they had groove sizes around .266 / 7 . The best bullet was the Hornady .264 160 RN . The .268 bullets caused too many problems . My three best shooters were 1. M-41 , 2. M-91 LR , 3. M-38 Cav carbine [ of all things ] . Most of my 7.35 rifles shot better than the 6.5 mm's . I sure wish they had made a M-41 in 7.35 mm . This was from about a 40 rifle sample .