it's isn't inirtia either that allows the gun to come unlocked. The "gas system" that JL was talking about was the fact that the gun comes unlocked as the gas preasure of the shot decreases to a safe point and the locking mechanism unlocks. Inirtia and the shooter moving forward after being displaced backwards from the recoil of a 3.5" is whats causing the chamber to open up slightly.

I had the same concern of my 870 super mag so I shot it off a bench without my hand on the pump. magically it stayed in the forward position.

Also not all gas operated guns have gas tubes. Just those designed to really cause all kinds of fouling in places you don't need all kinds of fouling (ie the action)