If its a 308 as appears by the mag there was something one of the guru's here pointed out with regards to the max weight projie and loading to be used in the MKIII conversion. May have been Bruce From Oz posted something about it with the MKIII Ishie's. Perhaps the OP of that information may chime in as I think the No.4 action is a bit stronger than the MkIII and can handle the higher pressure of the NATO cartridge but its still pretty high for the action.
Very nice build by the way a range report of course......