How much do I need to take off?It's not a very critical dimension, so you could just trim it with a Dremel and a file. I've done it by hand when modifying them to take a Wyatt's mag box before and it's not tough if you take your time.
How much do I need to take off?
I'm running CIP AICS/Accurate Mags....3.850Depends on the length of the box or mag your using. But like yorke said it can be done by hand. Milling is faster but not 100% needed.