This is the one. - Wilson Combat Rifle Length Gas Tube

I am using these tubes for a couple builds using Aero M5 upper, Aero BCG, and a Faxon barrel. Another reviewer stated it was too long for an Aero M5 upper, ha! Due to Faxon being a little sloppy with the gas port location, this is the only one long enough to reach the middle of the cam cutout. Most manufacturers are selling AR15 length tubes (roughly 15.2", depending on manufacturer). They can work, if the gas port location is right, otherwise, you're building a single shot rifle. In my case, a "standard" (ha!) length tube barely reaches the front of the cam cutout, and the gun cycles too fast to strip the next round from the magazine. I will be buying a few more of these to keep in the stash box.