Moonbogg wrote:My venom cannon has a flat back end on the chamber and it hasn't broken yet.
That's a bit different. Axial stresses in a pressure vessel are half those of circumferential stresses and there's also no hard corner for cracks to propagate from (unless you got the weld on the internal side of the join ground down to a sharp angle).
Still nowhere near as strong as the rest of it, but not as much of a problem as using your typical aluminium square tube stock as a pressure vessel.
Of course, in really high pressure vessels, you'll get no flat faces at all, and get hemi-spherical ends instead.
In the interests of a little extra safety, I occasionally modify flat endcaps that are going to be used at higher pressures so that the internal surface is hemispherical and better distributes the load. (Usually with the judicious application of epoxy putty.)