Just got mint old model SBH, made in 1968. Problem, decided to sight in at 40yds. Ran out of adjustment, back site bottomed out. I'm shooting 21 1/2 gr. of 2400 / with a 240gr JHP. Is this common on this model? I have another one that I bought new about same time. Shoot same load, gun is sighted for 100yds. and has never been touched since purchase. Other than trying to build on front sight the only thing I can think of is using lighter bullet or lighter load. Anyone run into this?