You can also use a "blow by tube". It is just a pllug with a hole in it with a tight fitting tube ran into a jar of water. It can burp but it cant burp back. If you use a big enough jug you won't need to worry about making sure your airlock stays full of water. Put a little sulfite in your water jug too, if you go that route.
Barometric pressures can make a solid bung pop out just as it can make your wine rise up into the neck of your carboy and up into your airlock in extreme barometric changes. Ive seen it on mine a few times when storms where coming in.
I have wondered about using the caps that come on our drinking water bottles.We have to purchase our drinking water in fives and always have them on hand. New, they cost .25 cents. They are a great seal and I have thought that they could be used for long term ageing. You would never have to worry about fluid levels of traditional stops.
What do you all think?
I have only bulk aged one wine longer than 6 months and when I did I replaced the airlock with one of these caps and kept it on for another 4-5 months or so and it seemed to work well. Once any off gassing has finished these should be okay; however, I'm not sure what atmospheric or temperature changes would have on one of these - perhaps blow it off if it there was a dramatic pressure drop. Others with more experience in long term ageing might offer some good insight.