Makes attachments duplication non profitable for metal farm (#5408)

# About the pull request

You can take M41A from weapon vendor, strip it for attachments (stock
and ugl), then restock M41A back and repeat those action.
Its 4000 metal from 2 attachments (or ~5 second for 1 sheet), then going
to be just 200 for both.

# Explain why it's good for the game

No more stacks of metal from nothing

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
code: Changed value of metal you are getting for recycling attachments
via autolathe.

/:cl:

code/modules/projectiles/gun_attachables.dm

@@ -31,7 +31,7 @@ Defined in conflicts.dm of the #defines folder.
 	var/pixel_shift_y = 16 //Uses the bottom left corner of the item.
 
 	flags_atom =  FPRINT|CONDUCT
-	matter = list("metal" = 2000)
+	matter = list("metal" = 100)
 	w_class = SIZE_SMALL
 	force = 1
 	var/slot = null //"muzzle", "rail", "under", "stock", "special"