As stated the rifleman's rule is an approximation, a rule of thumb; easy to remember and apply. I believe it originated when the typical scenario was using rifles at long ranges using open sights. In that environment, the sight line to bore distance becomes an insignificant part of the equation.
The problem with it is it isn't a good approximation at short ranges and with high sight lines, both of which apply in airgunning using scoped rifles.
Having thought about the bendy barrel thing, to be fair I think this is not so worthy of ridicule; it is something that might have a small effect. It's certainly not the cause of the issue but a free floating barrel will flex very slightly under its own weight. I remember shooting my HW100 with and without the moderator and the POI was an inch higher at 30 yards without the mod.
Last edited by Adam; 29th January 2016 at 01:30 PM.