If you only have one or two MLs then no problem. But most of us in here have quite a few. And except on a rare occasion do 2 rifles shoot the same thing the best. Keeping some sort of record is the only way you will keep things straight.
In fact, I keep records for all my CFs too.
Normally my records consist of nothing more than a spiral notebook with different load combos for each rifle showing powder, bullet, primer (if applicable) and group size.