I’m using an old Melvico prototype that was built back in 1995. It still works although it doesn’t have a gauge on it.
If you were to look at the parts, you might think: “Hell, I can build one of those.” Which is pretty much what I say when I see all the others on the market.
You basically need three ball valves: One for the gas in, one for the beer in and one overflow/release.
You start by hooking up the same CO2 source to the keg you want to bottle from and the gas in of the filler.
You then purge the bottle by letting in CO2 and letting “air” come out the release. After a few seconds, you close the release until the bottle is under pressure. You then close the CO2 and open the beer in valve while slowly releasing pressure. The bottle (hopefully) fills with very little head until full. You then release slowly all the pressure and cap immediately.
I’ve had mixed results in fill level but I’ve enjoyed beers, especially stronger beers, a long time after bottling.