Alan's Brewery
Mini Keg Bungs
There are two types of bungs available for the mini kegs: a red one for gravity feeding the beer from the keg's built-in tap; and a grey one for using with the CO2 driven tap.
The red bung
In spite of being a more complicated design than the grey bung, the red bung costs half the price of the grey one. It has a valve in it which must be opened when dispensing beer in order to prevent an airlock in the keg. This allows air into the keg, which can cause oxidation of the beer if the beer is not consumed fairly rapidly. (Personal experience has shown that the beer need not be drunk as quickly as recommended by the clever people, but I would try to finish it within 10 days.)
The red bung is made of a very hard plastic, and a fair amount of force must be applied to get it to seat properly in the keg. This hard plastic also makes it extremely difficult to remove, once the keg is empty. I tried using a pair of bent-nosed pliers to try to lever it out, but that only worked up to a point. When I tried to pull it out with a normal pair of pliers, the plastic shoulder tore. I eventually pushed a scriber through the plug and levered it out with that.
I definitely will not be using this type of bung in the future. I will rather modify the core plug of a grey bung for the situation where I want to dispense beer without CO2.
Update 24 August 2015
I had just as much trouble removing the red bung from the second keg as with the first. In hindsight, I have a feeling this may have to do with the hole in the keg being marginally too small (see the discussion under the grey bung below).
The grey bung
The grey bung consists of two parts - the bung itself, which is a fairly pliable rubber; and the central core plug which is made of a hard plastic. This central core is pushed into the beer with the CO2 dispenser, when broaching the keg. The core plug makes the bung fairly rigid, which means that a fair amount of force must be applied to get it to seat properly in the keg. However, it is nowhere near as difficult to seat as the red plug.
The bung comes out of the keg very easily with the hard core out because of the pliability of the rubber. The core plug was another issue...
The theory is that the core plug is shaken out when the keg is empty. However, nothing follows the theory, as the diameter of the core plug is marginally bigger than the hole in the keg (keg hole diameter: 2.175cm; core plug diameter: 2.260cm). When pushing the bung into the keg, you do not even feel the resistance due to that small difference in diameters, but when trying to get the plug out, that difference is a real pain if it does not land conveniently on the hole.
What I have done to the core plugs was to sand off a bit of the shoulder with a very fine grit emery paper. This allows the plug to fall out of the keg very easily.
Update 22 August 2015
I found that by seating one side of the grey bung first, then pressing the rest of the bung in, working in a circular motion, made it clip in very easily.
Update 17 July 2017
I no longer have the strength in my thumbs to seat the grey bung, so I had to find another method. What worked was to use a piece of wood, which I use as a sanding block, and press the bung down using body weight.
Epilogue
As a conspiracy theory, Fass Frisch probably don't want you to reuse the mini kegs, so the hole in the kegs has been made too small to easily remove the bungs!