My favorite load for the 6.5X57 is 46.5g of RL-19 with a Nosler 130g Accubond, RWS brass, CCI-200 primer seated to 3.055. Under MOA and deadly on deer. I have found about 50 to 100fps more than the 6.5X55 is the norm. Rifle is a Steyr with a 23" barrel.
I load 47gn H4350 with the 129gn Hornady in my 6.5x55 so 47/48gn should be ok in the x57. I guess the length of the throat will determine things. I suspect the reamer will have a long throat like the 7x64 as that's how the Europeans liked to throat their chambers a hundred plus years ago. I'm still waiting on the reamer to arrive.
John. I forgot what a great little action the ZKK is. And I have one in .30-06 with a custom stock that has sat up the back of the safe for a decade. I was going to build it on a Mauser 98 but had another look at the Brno. I wanted an action to match the cartridge. A 6.5x57 on a Rem 700 just wouldn't feel right.
Thanks Rusty. You must be a little short guy....................as things seem to go over your head.
