[tunabreath]

Hmm, Skladfin, you think maybe it was actually changing out the hop up rubber that saved your Five-seveN?

The only part I can think of that exerts significant force on the inner barrel (enough to shear those lugs on the inside of the hop up unit) is the loading nozzle when the blowback chamber is filling and pushing the slide back, and that's pretty much all transmitted through the hop up rubber.

Or is there a shoulder on the hop up unit that the blowback unit pushes against?

Either way, it's almost certainly a forward force on the inner barrel causing the damage both to the inside of the hop up unit and the stretching of the outer barrel (as the inner barrel strikes the shoulder on the inside of the outer barrel).

I'm not sure about the pins on the blowback unit... Those would only be stressed by the slide being lifted as it blows back, yeah? Note that the blowback chamber is still held very solidly in by the screw to the rear sight and the plastic moldings inside the slide. It was just the rails that were loose.