After approx 2000 rounds and no previous problems with the gun whatsoever, a casing would not eject whilst shooting from the bench at the range.
Upon returning home and pushing out the shell casing from the muzzle it became apparent that the extractor had snapped off clean approx. 3/16" from the hook end.
Is this a common thing?
Are there stronger aftermarket items available?
Thank you for any replies.

Give armalite a call and just have them replace the broken extractor.

As for common, no not in only 2000 rounds.  It may have just been heat treated too much, or had a stress crack that was not seen.

Sometimes things just happem, So I won't loose any sleep over it.