No. They have curved edges, so they can't be polyhedra.
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This is for stability.
Two cylinders with the same volume are not necessarily congruent. One could have a different diameter and length than the other, and still have the same volume.
729:1
6 to 1
There are a number of possible 7-sided polyhedra. Three that come immediately to mind are: a hexagonal pyramid, a pentagonal prism, a "stretched" triangular pyramid (triangular base, rectangular sides, and then triangular faces meeting at an apex).