I'm not sure if this post should be here or in Armor/AFV, but anyway...

I'm replacing the barrel on an AT gun, essencially scratch building the replacement barrel...
my questions are:

• what constitutes the start and the end of the barrel?
  
• the real barrel measures 2564mm, in 1/35 does this mean I should simply divide the 2564 by 35? Does this mean my scratchbuilt barrel should be 73.25mm long?
  

thanks

As an ex-gunner, my understanding is that the barrel begins immediately in front of the breech block and ends at the muzzle (including any muzzle brake fitted) - ie. the total length of tube that the shell travels inside the weapon.
As for the second question, then yes, divide the real piece by 35.