Booligan Airsoft goes for HPA to power his Stubby M4 Pistol and uses the Wolverine Airsoft Inferno Engine. Here's a quicj demo video... "Last week, I said what I'd be doing with my stubby M4 pistol. Today, let's do a quick demo to see how it shoots! The Wolverine Inferno system installed easily and the Storm InGrip regulator lets me run CO2 instead of HPA. I'll be posting another, more detailed video soon showing the pros and cons of this setup, but for now, I'm loving this ugly little thing."