Just messing around in a custom game and my plasma pistol started charging and I have a scuf with trigger stops. As far as I knew, this wasnt possible. So I figured out the buttom combo that makes it charge. Not the most graceful, but it works none the less.
Button Combo:
Press right trigger and hold it down (it will shoot the plasma pistol once), while still holding the right trigger down, slide your thumb up the controller in a fast motion sliding over the "A" button (keeping holding it down) and then keep sliding your thumb up until you hit the "X" button (at this point you are holding both "A" and "X" down at the same time), the plasma pistol should start charging shortly after (maybe about 1 second). You can then release the "A" and "X" button, still holding the right trigger with a fully charged plasma pistol. It seems there is some timing required like double shoting when going from the "A" button to the "X" button, but its not hard. If you dont get it on the first try, start over firing the plasma pistol again.
My settings are set to defaut so "A" is jump and "X" is reload for those with different settings.
Hope this helps!