@splash27: I get the same result - but I see you have converted it to Blender 2.80. I’m having some trouble with Bender 2.8 so I will see if I can fix that. Will also try to look with more detail at your files.
It does seem that the .babylon file when loaded into the sandbox tells me that the mesh is there and its material - but is not showing up. Right now, I have no idea why!
cheers, gryff 
@splash27 : Well I had some more time to look at your Blender 2.80 file and the disappearing “hands” mesh. I don’t have an answer but found a few things that seem strange.
there is no references to textures for some reason
I’ve no idea how you got from the 2.7x file to 2.80 file but first thing I noticed was opening the “shading panels” then looking at all the mesh materials (see image 1). No cycles or evee materials, and note the box “Use Nodes” is unchecked for any mesh, although something gets exported. Probably explains the lack of textures.
Now as for the missing “hands” mesh, I did the following things:
Opened your .babylon file in the sandbox and the “hands” mesh is there (left panel, Image 2), so in the right paneI I activated “wireframe” - the “hands” show up! I then selected the metarig as before and brought up the animations - the animation plays BUT the “hands” mesh does not move. (see image 3)
So I looked into the .babylon file at “hands” and I see this :
a. for the original .babylon file I sent you:
{“name”:“hands”,“id”:“hands”,“materialId”:“HK416_1c.v_hands.vtf”,“billboardMode”:0,“position”:[0,0,0],“rotation”:[0,0,0],“scaling [1,1,1],“isVisible”:true,“freezeWorldMatrix”:false,
“isEnabled”:true,“checkCollisions”:false,“receiveShadows”:false,“tags”:”",“skeletonId”:0,
“numBoneInfluencers”:8
b. but for your latest 2.80 export:
{“name”:“hands”,“id”:“hands”,“materialId”:“v_hands.vtf”,“billboardMode”:0,“position”:
[0,0,0],“rotation”:[0,0,0],“scaling”:[1,1,1],“isVisible”:true,“freezeWorldMatrix”:false,
“isEnabled”:true,“checkCollisions”:false,“receiveShadows”:false,“pickable”:true,“tags”:“”,“skeletonId”:0
,“numBoneInfluencers”:0
So that mesh coming from Blender 2.80 has no bone influencers. No idea why this is happening as both the exporters are set to a max of 8 influencers .
I don’t know how you created this file - but there do seem to be issues (certainly with the materials).
I might also note that when you deleted those unused bones - a lot of empty vertex groups got left behind. Impact of all this … ?
cheers, gryff 
@gryff, I just opened your .blend file in Blender 2.80, removed unused bones, animations and also silencer mesh, added hide_unsill animation and saved it. Then I exported it with babylon exporter 6.2.1.
So, all my 2.7-2.8 conversion is just opening your file and saving it in Blender 2.80.
