I thought that thread would have given you a good place to start, it should have done, but I'll show you how to find out the information anyway.
The MiniGun lives in xWeapons\Classes, in its defaultproperties you'll find this code:
These two lines define what happens when the player fires the weapon (either primary or secondary fire). MinigunFire also lives in xWeapons\Classes, and by opening it with WordPad you'll find that it extends InstantFire. Surprise, surprise; InstantFire can be found in xWeapons\Classes too.
As a start to UScripting you should have d/loaded the official Epic source code, which is available for the current patch of 3270. Just by looking at what's contained in the UC files you'll see how the original game was constructed, it may be confusing at first, but that's all part of the learning-curve.
To get the new weapon to fire rockets in the secondary fire-mode simply change its FireModeClass(1) to RocketFire, you may have to do some extra coding to get the results you want but in a nut-shell that's all it's going to take.
Hope that helps.