I have an old Sheaffer Targa matte black rollerball pen that I really like. I got it at an estate sale. It came with an old dried out refill. When I looked for replacements, I was not happy with the cost or color choices. So I decided to do some modifications to the pen barrel....
Step 1: Messy But Worth It
Using a dental pick, I scraped down into the barrel, removing the hard plastic ridge around the inside at the bottom. Then I carefully scraped and pulled until the small spring popped out. After more scraping, I was able to fully insert my preferred refill.
Step 2: Success!
After removing the extra plastic and the spring, I was able to screw the top all the way down.
Step 3: Schmidt Refills
Schmidt safety ceramic roller refills 5888 M - they come in a rainbow of colors and are very smooth.