I looks like you have the same issue I had. The issue was in the Weapons file (the new/converted weapon) or the Items file for that weapon. I use XML notepad as it is a good way of checking the structure at a glance. Other things I noted was the occasional missing "/<Item>" or "<Item>" due to copy/paste errors or (although it should not make a difference) on the same line... but then the using notepad++ you can sometimes get them under the wrong cascade. I take it you are getting everything you want in the IMP item choices to appear but not the MG itself? It really is useful to know if you can get a 100 round belt of 3006 to appear in your selection. If you are let me know and I will see if I can either replicate the issue or make it work.
Regards
Ratty
Oh, one more thing, the editors keep a temp version of the last file edited and its important to close all but the one you actually want to edit or you can end up overwriting amended files. Usually it will give you a warning about reloading the changed file but YAMMV.