One of the greatest personal incentives to me is having a mechanism that replicates that of a firearm, long time members will tell you of my fascination with shell ejection

Some other practical advantages include the possibility of a repeater with irregular projectiles (such as a shotgun), having the leak-prone pressurized mechanism restricted to an easily replaceable cartridge as well as the opportunity to create a variety of different cartridges that can all be fired from the same launcher.
In the meantime I've crunched some numbers and figured some practical mechanics, here's what a high pressure exhaust valve cartridge wold look like for a 3/8" ball bearing, 0.75" diameter and 3" long making it roughly the size of a shotgun shell.

The front end is permanently pinned in place while the rear is removable for maintenance purposes. The piston is machined from a single piece of delrin, and I envisage filling being done through the mouth of the cartridge via an appropriate fill rig. Not shown is a groove or rim for extraction purposes, I'm still debating if this will be necessary - technically it would be easier to extract a shell that for whatever reason didn't fire, but the same can be accomplished by pulling the bolt back and letting it fall out by gravity - not exactly elegant engineering, hmmm...