1 (one) x (times) anything is.....one.
Been in the aircraft business for over fifty years.
2x to 3x works well for most common sized rivets. Anything rated higher is like driving brads with a sledge hammer. Kinda hard to control.
So true. Most general aviation riveting can be done with a 2x and a 3x. In the airline business we make good use of a 4x. Nowdays, larger stuff is hi-locks, they don't use the huge rivets any longer, as a general rule. (7x and 9x are great for beating stuff apart however

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Most rivet guns are either Chicago Pneumatic or exact clones of them (the design patents are long ago expired). I have a very old, nearly new, Cleco brand (made when Cleco was owned by the Reed Roller Bit Co.)
If you can find a Cleco 2x or 3x
in this style (but with the shorter barrel of the lower power gun) you will like it, they have the best feel and handling of any I've seen.
Beyond that, get a genuine CP. At work the tool room has tried all the clones and they say they never held up as well as a genuine CP gun.
Charles