Ok here is a little and simple guide how to have unlimited Heavy Weapon ammo. First you need the program Mass Effect 2 Coalesced Compiler for easy editing. Her is the link (Link), the best would be to use the zip file so you dont need to instal the program joust unzip it and run it.
Ok so now that you have installed or unzip the Mass Effect 2 Coalesced Compiler go to the folder you unziped or installed the program find the Configs folder and there you will find alot of .INI files. The one that we need is the BIOWeapon.INI file open it with notepad or any other text editing program. Now what you need to do is to add (bInfiniteAmmo=true) without the () under the (bIsAutomatic=)line.
Those are the weapons you need to add the (bInfiniteAmmo=true) without the () and you will have unlimited ammo for all heavy weapons. For the NukeLuncher you need to reload after each shoot but still you wont run out of ammo, and dont worry if the ammo counter shows -1% it works 100%.
If you want unlimited ammo for all other guns joust find the line [SFXGameContent_Inventory.SFXWeapon_GarrusSniperRifle] and change all the weapons that are abow the line bInfiniteAmmo=false to true.
Ok once you change everything you wanted close the notepad and save the change. After you close it run Mass Effect 2 Coalesced Compiler and press Compile Coalesced.ini close the program run the game and try if your done everything right.
Stupid question I'm sure, I purchased the Steam Version of the game, this means I cant do any direct file editing correct?
No, you can edit the ini all you want (I should know, I've modded this steam game, and many before it)
All you have to do is download the Coalesced compiler linked in the original post, make your modifications to the ini files, then hit compile, and it will replace the stock Coalesced.ini with your modded one.