A question about CO2 purging

I've been using a Blichman Beer Gun for a little over a year now and a thought hit me this weekend while bottling up around 8 cases from kegs. Why is it a good idea to purge the bottles with CO2 before filling? You hit the bottle with 1 - 2 secs of CO2 to force out the air, fill with beer until clear beer runs over the top, remove gun (drawing in air) and cap. Seems that all that CO2 you were purging with gets lost as you COMPLETELY fill the bottle with liquid. Am I missing something or have I been wasting a lot of CO2 for nothing?

Rick near Atlanta

You are displacing the co2 with beer, which is what you want. The whole reason for purging with co2 is that you don't want to oxydize the beer. If you didn't purge, you'd be diplacing regular air (part oxygen) with beer, so the beer will have more contact with oxygen. Some air will enter the bottle as you remove the gun, but that's when they suggest to give it a shot of co2 to purge the headspace and then cap.

BrewThruYou said:

Some air will enter the bottle as you remove the gun, but that's when they suggest to give it a shot of co2 to purge the headspace and then cap.

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Or, you can hit the CO2 trigger with the tip just under the beer surface causing it to foam up and then cap on the foam.

Many thanks. Clears that up for me.