Quite possibly the most popular disposable pens on earth are Pilot G2 gel pens. They are so incredibly smooth and fluid for any type of ball-tipped writing device.
About a year ago, in an act of refill desperation, I was out of standard rollerball refills and trying ship out a pen I had just sold, in which I promised a fresh, working rollerball refill. That is when I noticed that I had some spare Pilot G2 refills lying around, which I had gotten and ignored from a large collection I had purchased.
On closer inspection, I realized that the two refills were close enough alike that a G2 might fit in the Waterman that needed it. Lo and behold, it did. It wrote magnificently! Pretty soon I was popping them into Auroras, Pelikans and every other rollerball that needed a similarly sized refill.
Not all pens take them easily. As you can see, this Pelikan replacement refill is a little longer than the G2. BUT, if it turns out the G2 is too short, just wad up a little tissue and fill the back end of the pen with it until the G2 fits and writes without any jiggling.
I like rollerball refills just fine, but none compare to a G2…for me…in the writing experience. I don’t see using regular rollerball refills, again.