This might be a bit too much for the start but it holds pretty much info on guns, and with some patience you will be able to make the gun of your choice with it. Its a bit old (last update on it was a year ago).
a few words for the start:
get and install the FL SDK 1.3, you can get it from lancersreactor's downloads.
install it to a directory somewhere.
this is hte best reference that you can find - it has all freelancer inis in an uncompressed and corrected format. so to say the base of all mods.
you have FLMM. goto the FLMM directory and browse to the "mods" directory. these includes all mods that are installed on ur PC.
now copy the folling text to a file and rename it to script.xml:
<script>
<header name="My testmod">
<scriptversion>
1.0
</scriptversion>
<author>
Me
</author>
<modurl>
mywebpagelink
</modurl>
<description>
mydescripton
</description>
</header>
</script>
edit this as u like - fill in ur name, what the mod does and so on
now create a the directory called DATA inside the mod's folder.
inside DATA, create an EQUIPMENT folder.
now copy the file weaponequip.ini from the freelancer SDK\DATA\equipment folder into our Equipment folder.
search for the string "li_gun01_mark01" (without "
this is the justice MK I
now read the tutorial from the link. it will give you all info to change the justice MK I into the gun you want to have.
you can also make a new gun from scratch.
so far for now, let m kow if you got lost somewhere.
Fjord