The other big issue is that in "the good old days" folk were tuning their rifles for a specific ammunition: Mk 7 ball. There was no other authorised ammo. In many ways, that made it easy, as what worked for one would probably (possibly?) work for another.
If someone is tweaking a SMLE these days, it is unlikely that it is for Mk7 ball.
However, what might work for bullet "A" at seating depth "B", propellant quantity "X" of type "Z" and primer "Y" in case "K", may not necessarily be applicable to any other combination. And that assumes a barrel in near-new condition.
You can go through a small fortune's worth of ammo finding a load that suits your rifle; a process that can be entertaining and informative. Or maybe you can get lucky at the "ballistics casino". However, if you run an ordnance system that holds a couple of million rifles and machine-guns, having a standard round at least gives you a reference point.
What we need is a new source of Mk7 projectiles.
