1. Create a new folder in your FS2 directory. Name it "mymod" or whatever. Place all your custom data in mymod\data\whatever, following the rules as laid out here:
http://www.hard-light.net/wiki/index.php/FS2_Data_Structure2. Create a new text file in the mymod directory. Rename it to "mod.ini". Copy the following text into that file:
[multimod]
secondarylist = mediavps;
Save the file.
3. Use the Launcher to select "mymod" as the active mod. The game will now load data from mymod before the mediavps, enabling both mods.
Now, none of these issues help you find the issues directly, but they make organizing your modding work easier, and as I said before, makes helping you once you run into problems easier.
It seems to me that you should spend a good deal of time absorbing the info in the Wiki's
modding portal, especially with regards to tbms.
As for your current problem, first see if the issue appears with the unmodified mediavps. If it does, we'll work from there. If it doesn't, post the weapons' tbl entry, so that we can take a look at it.
EDIT:
WARNING: "$Rearm Rate for ballistic primary CMI 71-KEC not specified. Defaulting to 100..." at Weapons.cpp:2585
Int3(): From e:\storage\fso\fs2_open_3_6_10\code\globalincs\windebug.cpp at line 1326
This is one error in the weapon entry you _need_ to correct.