agentzero wrote:100m is sort of "the usual" for sighting in isn't it?
Yes and no.
100m is basically the shooting "standard distance" for most things unless otherwise specified.
e.g. If someone says they're shooting a 1" group with their rifle, unless they specify otherwise they're likely talking about shooting at 100m.
Same for sighting in - 100m is "the norm" before setting up for your particular situation.
The reason to sight in at 200m instead of 100m when hunting is that it makes shooting over a varying distances easier because there is less drop to factor in when compared to sighting at 100m.
Expanding on what was mentioned above, using some of my ballistics for a 168gr 30-06 load you get the following:
When sighted in at 200m you'd be somewhere around 2" high at 100, bullseye at 200 and about 8" low at 300.
If you were sighted in at 100m with the same load, you'd be bullseye at 100, but 4" low at 200, and 14" low at 300.
As you can see, less correction is required to stay on target over those distances with a 200m sighting. Pretty much just aim slightly above centre mass at any distance and you'll still get a hit on target.
For a 100m sighting you'd have to be constantly thinking about your drop more and varying shot distances.