If the cannister burst in the gun tube, the iron balls would ricochet around the tube, loosing energy and damaging the bore of the tube. Also the main thrust of the expanding gas from the propellant charge would act on the wooden sabot, and then iron plate at the base of the canister, and act like a plunger, shoving the cannister up the gun tube.
Austrian cannisters had a weak point in their design, specifically to allow the cannister to break. The percusive force of the propellant gas acted on the iron base plate, the V or valley fold around the tin cannister, would crumple, foreshortenning the cannister, which in effect knock the top of the cannister. I think.
We can guess at what happens to the cannister in the tube, but dont know what happens as no one, as you say has x-ray vision, can see what happens to the cannister and any definate 'I know what happens to the cannister in the tube as hard fact' is hyperbole, because as yet no one has provided the evidence of what actually does happen more than speculation.
The upshot is, in 200 years we still dont know how cannister actually works. This is also true about a lot of the period. How did the guard infantry with four company battalions form square as the 1791 regulations dont really work for four companies.