Thanks Steve004 . 

That would make sense . An obstruction as the cause . You’d expect this observation ( ring under the front sight )  then to be more  geographical to a degree  ( environments that experience snow ) .  Obviously over time , some of those rifles may have been into climates without  snow yet still bare the ring from being fired when once obstructed by snow .

Arthur Pirkle ( if I’m correct ) only references this in his comments towards 1892 barrels & not other models ; by way of explanation why some barrels may have been shortened in the hope  to correct a loss of accuracy . 

 Seems peculiar if limited to 1892 barrels . 

Your right Steve004 ; dovetailed or sweated the front sight on – you’d expect to see a correlation between the ring developing under the front sight & the method used to install the front sight if that was the cause . ( ie manufacturing ) 

I get it , that other types of obstructions could have similar results .

Which lead to a  brief thought , regards possible unreliable or handloaded ammunition back then  ( maybe regards black powder & it’s storage  ?) .   A projectile not exiting the barrel & a subsequent shot being fired being the cause of the ring . 

However , as 32-20 & 44-40 were common to other rifles ; you’d expect to observe the same incidences in other rifles of the time as well . 

Thanks for your thoughts Steve004