The rifle I am talking about has an 'S' stamped into the barrel. I think this means it was re-sized after WW 1 to take a longer round.

During WW 1 dum-dum rounds were used, and somewhere along the line the major nations agreed to never use them again in combat. This led to a mass re-sizing of the bore where the bullet enters it to make is slightly, like 2mm, longer.

As weird as it sounds, the British did this to their .303 and then made millions of rounds that had a bit of sawdust in the tip of them. The lighter and less dense sawdust acted just like the hallow point of a dum-dum when it came to hitting a target, but since the rounds did not have a hallow nose they were 'legal'.

It probably took some Lawyer to come up with that.