Don't bother with a stock replacement, especially from Milwaukee.
Buy a Rohm.
Pick the size you need and be super happy. I have swapped every drill I own to a Rohm and will continue to do so on any others. They just feel so much better than the stock nonsense. I have them on both Milwaukee and DeWalt drills (12v and 18v) in both 3/8" max and 1/2" max respectively.
You can search for DeWalt Rohm (they use Rohm chucks on their higher end hammer drills) on the auction site for a great deal on an excellent chuck.
