Modding Impossible?

Greetings:

I've been trying to alter the Weapon Effects on my current weapon, since the muzzle fire effect is far to big.
I managed successfully to alter the muzzle fire effect, I save the file, go ingame, launch, seek a target, fire... and nothing has changed.
The same huge flashes are being displayed.

I am in the correct file, I am on the right weapon, I made the right change, but when I re-open the file, all changes have been reverted.
Apparently, when I take permissions from System to alter the file, the launcher simply stops on me.

How can I stop these reverts?
Or alter the 'original' file? (where is the darned thing anyway?)

Thank you beforehand.

I am so very much aware of this, but I am not online, my friend.
I am running the single player run-through.

Thank you for the response.

I understand, but the question was if there was a way to bypass this in my case?

Feel free to answer in a PM, I do not want others to have this info, logically.
IF you want to answer this one, of course.