Nice read. I like old stuff
Regarding the BSA being off sideways and the Webley only vertical etc; you cant really conclude it definitely is the rifle. There are so many other factors to consider.
Zero position > BSA always straying to the right may suggest zero postion error
Trigger; weight and LOP > more weight may cause trigger errors and/or cause more pressure on the pistolgrip. LOP same
And, of course, because of different barrel harmonics per rifle (due to different barrel length, speed etc) the pellet will have a different POI anyhow between 2 different guns


That said; every airgun is different; not all like all stances. I had a NEW Diana 31 which refused to group in sitting FT position. I refused to believe there was something wrong with it as it was new so started experimenting with my hold/stance. Eventually shot it from the fist and it became my most accurate airrifle so far (springer) with 7mm edge-edge groups at 20m.

Must add my MK3 seems to be quite forgiving in hold