It's actually very easy to do. The bottom of the mag has a removable floorplate. Remove the floorplate and pull the spring and guts out the bottom. I used a chainsaw file and Dremel with carbide burr bit.
1) stack 2 rounds in the mag and mark on the with a sharpie where the tips are. Then remove rounds.
2) Remove bottom floor plate of mag and then remove guts.
3) Now the mag should be gutted and can see through
4) VERY IMPORTANT - There are small rivets on the front end of the mag that holds the aluminum in place. You do not want to damage the rivets, because they hold the entire internal side and front piece.
5) File in the area where the line is most of the way down. This will allow the stacked rounds to clear as well. You'll notice how deep you need to go depending on how many the mag holds.
6) Once the groove is made, then I used the Dremel. Making sure to stay away from the middle where the rivet is.
Once I thought I had enough, I would blow out the shavings, put it all back together, and see if the rounds fit. If all 3 wouldn't, I'd go a little deeper.
My action measured 3.817" but my COAL to lands with the 162 eldms was 2.730". You just need to make sure that you don't Dremel it all the way to the outer shell because it's all one piece.
Got a little more length