MOST pistol caliber uppers are blowback, but not all. Google 9mm, 40S&W, 10MM, 45acp, 357Mag, 357SIG, 44mag, 45WM, or 9x23 DI uppers and you should be able to find info on them. I have 3 blowback 9mm's and 3 DI 9mms. The 9mm DI barrels have a gas port about 1.25" in front of the casemouth step in the chamber and use a 5.45x39 BCG (or opened up 223 bolt) I mostly shoot the blowback guns as they're cleaner, and less picky about ammo.