I like to think that the break in period is really you getting use to the rifle. I have several rifles ranging from light, medium to magnum. As each rifle and persons are different so bear that in mind. Next is based on how it works for me
Light duty - easy to manage - Remington Tyrant .177
Medium duty - are always hold sensitive - Remington NPSS .22
Magnum - I'm holding it tighter than normal - Hatsan 125TH .25