When switching bullets, powder, primer, cases or when accuracy drops off.
The reason for including the powder, primer, cases is potential pressure increases. Obviously when you change bullet types you would want to check Ogive length for that bullet type. I have a buddy that checks ogive for every box of 100 he reloads. When he opens a new box he checks the length then loads those 100. Overkill but it works for him.
Sorry, what is a jam measurment?