Since we are now on to the subject of if adding a mag sled is considered as conversion or manufacturing, I offer you the following reading material from BWeise.
Now, I'm not saying what he say is gold. However for what it's worth, BWeise stated that any parts that could be switch out during the normal course of field cleaning would be ok for a 01 FFL to perform. Keep in mind that since 01 FFL is not licensed to manufacturing firearm, it would seems BWeise is implying either adding a mag sled or switching barrel would not be constitute as manufacturing a firearm.
http://www.calguns.net/calgunforum/s...d.php?t=441766
http://www.calguns.net/calgunforum/s...d.php?t=442220