[ThunderCactus]

I know bearings very well, I fix CNC machines so I really have to know a lot about them

I'll let you in on a trade secret, most of the really high machines use 'sealed' greased bearings for ballscrew bearings, tailstock bearings, live tooling bearings, horizontal and vertical spindle bearings.
They are open bearings in packs, but mechanically sealed by cover plates, etc.
Anyway, they last about a decade depending on what you're doing, and they're not "packed" per say, we only load them up to about 15% of their internal volume with grease. So you don't need that much. And that number becomes VERY critical in high rpm bearings. There's been many cases where old millwrights have packed high accuracy bearings 100% full and had them fail after a month. It's because you can't pack them 100% and you absolutely need the proper grease.
Most common reason for bearings to fail are; mechanical impact (that's the big bearing killer), wash-out of the grease (due to seals failing and coolant running the grease out), angular run-out (which is like mechanical wear, running a load in a roller bearing on an angle it's not meant to handle), and overheating (which causes the grease to lose it's lubricity and also decreases the tolerance in the bearings.)

That being said, the high end machines that use high rpm spindles, like 12,000-30,000rpm, use air/oil sprays to lubricate the bearings constantly, but because they are subject to heavier loads for their size compared to airsoft bearings. They need better lubrication.

And airsoft related, what kind of bearings do you think are in the 20,000-30,000rpm motors we're running? They're all sealed, and the very few of them that are actually high end bearings (systema, eagle force, tienly) last a very long time.
With PTW's motors it's the same story, all sealed bearings. Most common bearing to fail in the PTW motors is the top end bearings, most commonly when the bevel is too high, causing heavy loading on one side of the bearing.
The open bearings that most manufacturers use, like G&P, just throw the grease out over time, and if you're dumb enough to use oil, all the lubrication is gone after the first game.

So ceramic sealed bearings will work great for the bevel and spur, but with the sector gear one would think bushings would be best. My concern is the repeated impact with the piston would cause a wear point in the bearing, though I have little evidence to support that. Oddly enough it's usually the spur gear bearings that fail...