When we started thinking about a small tactical pen we wanted to have a pen that is as small as the refill would allow but still have something that you could use to get out of a bad situation if you had too. So as you all can see we kept to the basic idea of all of our pens and kubatons with the deep grooves at each end of the body. This pen also has the same spiral grooves in the middle portion of the body as the Extreme Duty Spiral pen. So even though this pen is shorter and smaller in diameter you can still get a very good grip on it when striking!
The Investigator and Extreme Duty pens both take a fisher space pen PR refill.