Counter Pressure Bottle Filler - Beer Gun

How a Counter Pressure Bottle Filler Works?
The premise behind a counter pressure bottle filler is to fill the bottle while it is under constant pressure to prevent foaming and loss of carbonation.
 
The actual process takes place in stages
In the first stage, the bottle is filled with CO2 to purge all of the air from the bottle.
Second, the bottle is pressurized so that the CO2 in the bottle is at the same pressure as the carbonated beer to be added.
Third, the bottle is filled, and CO2 is allowed to escape from the bottle as beer displaces it while maintaining constant pressure.
Finally the filler is removed and the bottle is quickly capped.
 
If the steps are done properly, the beer will be under constant pressure during the entire process and will retain its carbonation.
 
Counter Pressure Bottling in Practice.
While the theory is straightforward, using a counter pressure bottle filler the first few times can be a challenge.
 
Here are some tips to help keep things under control
Keep the keg at serving pressure, and if possible use a filling line that is approximately the same as your normal serving line. This will assure that the bottle is carbonated at the correct pressure.
Chill the bottles in the refrigerator before using so there is no change in temperature when the beer contacts the bottle.
The bottles must be sterilized, which presents a challenge since they also should be cold. Fill the bottles with an iodophor solution before putting in the fridge, then dump before filling.
Make sure that the bottle filler is properly seated before filling.
Fill the bottles nearly to the brim, as removing the filler will leave you with the correct amount of headspace.
Moderate spillover when removing the filler is normal due to the pressure release before capping.
Cap as quickly as possible after removing the filler to minimize the chance of air getting into the bottle.