You're talking two objects mutually attracted by gravity, first converting potential energy into kinetic energy to force them apart, and coming back together, exchanging potential energy for kinetic energy and visa versa. Some matter is converted into heat energy due to the exploding gunpowder and some kinetic energy is converted into heat energy due to atmospheric friction. Taking into account these losses, there is conservation of matter and energy.
But this model is at odds with the observation that the universe is expanding and the expansion is accelerating. In your ballistics analogy, the objects start at rest and end at rest. The initial acceleration is due to converting the gunpowder to hot gasses and the final acceleration is due to gravity. The objects start together and end up together. In the observed expandiing universe, the objects are going faster and faster and getting farther and farther apart. Acceleration requires an input of energy. You can't accelerate an object without expending energy. Continuous acceleration requires a continuous input of energy.
In the shell example, there's an initial input of energy from the exploding gunpowder. Then as the shell arcs upward, it gains potential energy as it gains height above the earth. The kinetic energy is being converted into potential energy. There is no additional energy being put into the shell/earth system. When the kinetic energy has finally been converted to potential energy, then the shell begins falling back to earth and its stored potential energy gets converted back into kinetic energy thanks to gravity. Again, there is no additional energy input into the shell/earth system.
What's actually going on in the universe is totally at odds with your shell analogy. Together/together <> apart/further apart; at rest/at rest <> fast/faster; conservation of energy <> increasing energy; closed system <> ????????
I'm using the terms in the scientific sense; how far we can see the universe has nothing to do with it.
If my theory were true, then what we consider to be the universe WILL end at rest, when the gravity of it's point of origin draws it back to the cauldron from where it came. Completely regenerative, repeating itself every few billions of years.
Are you suggesting that gravity is not a form of energy? You stated that there was not additional input of energy, yet kinetic energy is generated by the application of gravity??
And how far we can see has much to do with it. To suggest the universe is a closed system is to also suggest it is contained. Contained within what?