@dadpad,
I'm surprised at how efficient an air pump (yes it's a larger version of a bicycle pump, that Mr. Girandoni made with the limited tools available at that point in time, though it was only meant for use in the field to top up the charge, rather than to fully charge the cylinders.
For combat purposes, a compressor unit charged the cylinders, which were carted in behind the frontline troops, and bought in by runners. The changing of the cylinders was simply screw off the old, and screw on the new.
With the advances in battery tech today (I'm still waaaaaaay impressed with my 18 volt grinder and impact driver and recipro saw) a small compressor unit is not that big a stretch. My little cheapo Chinese tyre inflator maxes out at 300 PSI, and it only cost twenty spondoolas at soopacheep. It's not that big a stretch to envisage an all-ceramic compressor unit that fits inside the rifle stock, alongside the battery, to supply the 800 PSI or greater.