I own and have killed deer with all but the 6.5X55. However, I own and have killed several deer with my 6.5X57's. I see no advantage the 6.5X55 has over the 6.5X57, and the 6.5X57 does not have the odd sized case head of the 6.5X55. The case capacity is virtually the same and modern loadings make the two practically indistinguishable. The 6.5X55 has benefitted from the number of former Swedish army rifles which once flooded the market.