If you like fine gel pens but would prefer a smaller form-factor, this instructable shares how to modify a 0.38mm gel pen cartridge to fit inside of a G2 mini pen barrel.
The original how-to was posted on my blog in Spring 2006.
Step 1: G2 Mini Vs. Uniball Signo Comparison
Here you can see how the G2 mini and Uniballl Signo compare side by side. The G2 mini is about 4.5 inches long, compared to 5.5 inches for the Uniball Signo. For pocket-ability, that reduction of an inch means quite a bit â making the G2 mini well-suited for pockets.
Step 2: Mark the Uniball Signo Cartridge for Cutting
Next, I opened up the two pens to compare the length of the cartridges, and as you can see, the Uniball Signo is about 1 inch longer, but has room for trimming. I've noticed that new Uniball Signo cartridges have ink at or above the location you need to cut them down without creating a mess. The easiest way to remedy this is by drawing with the cartridge until the ink level drops enough that a slice is reasonable.
Step 3: Cut the Uniball Signo Cartridge
Using a utility knife , x-acto knife or other sharp instrument, trim the Uniball cartridge down to the same length as the G2 mini cartridge as shown in the photo above (see the dotted line). I've that the Uniball Signo cartridge uses a much thicker outer wall compared to the G2 mini, so the G2 may actually have close to the same volume of ink even though it looks like less.
Step 4: Check the Modified Uniball Signo Cartridge
Here you see the nicely sliced Uniball cartridge, ready for insertion into the G2 mini pen body. If you want to be non-wasteful, you can keep the G2 mini cartridges for backups, or use clear tape to adhere the chunk of Uniball cartridge you've sliced off to the top of the G2 mini cartridge, and use this in a standard G2 pen. The G2 plus clipped cartridge combo doesn't work in empty Uniball Signo pen bodies, because of the G2's nib.
Step 5: Enjoy Your Mini Uniball G2 Signo
Use your new G2 Uniball Signo RT Gel 0.38mm mini pen, and enjoy! :-)