Robin I think i agree with Dave on this one. By taking the top off this tit that is less "over cocking".
By bringing the whole shelf down (small amount at a time) also decreases overcocking. This can be watched by checking bolt closing when the cocking pin catches the sear.
Taking the shelf down too much results in scrap bolt body (don't ask). This does not affect pin fall as pin measurement and sear to pin stop are not affected.
I'm sure sharpshooter will be glad to enlighten us on this again.��