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.

Sure do. I have too much other stuff running through my head to remember everything. I keep everything in Excel. I note everything as well that way I can sort them to find what I'm looking for.

For instance, say I want to see if a particual bullet shot well. Just sort it by that bullet and it will show the groups I've shot with it and with which gun. I also keep my targets as well which are numbered and scanned. Then entered into Excel for a visual if I need one.

Coming from a Business Analyst position my brain works best this way. A friend of mine just uses a notebook and that works great for him.

I am a rookie at ml but being in the Quality Engineering field for many years proved to me that record keeping is a must. Especially when you get to be older and the memory isn't quite as sharp.
As was mentioned you can use a simple Sprial Notebook and go all the way to using an excel spread sheet.

I don't but I should. My memory aint what it use to be.
Art

I do keep some of the "range targets", but frankly seldom refer to them. I do keep a note book in which I record all range information. Such information as bullet type, weight, powder charge, powder lot numbers, MOA performance, etc. .... and I do this for my ML, shot gun patterning, hand loaded center fire, as well as any factory loads that I try. This is my main reference.

I keep some of my targets and I also started a spreadsheet to keep track of loads and best group size.

There are photos on my laptop of all the deer i've killed since about 2002. There is just the date killed and no particular gun is identified. There are also photos of some of my hogs long with the date. Most of the hogs never get photographed.

I keep a file with all my range report info in them with pics & targets sometimes. I usually write up an actual "report" like I was in school for each long range session. If I just shoot a few times to check zero or get re-aquainted with the rifle, I don't keep records of that.

It's kind of fun (and helpful) to look back on previous range sessions and see what worked and what didn't.