Counter pressure filler question

For those of you who use the BierMuncher beergun (and any counter-pressure filler for that matter) is there not a concern about loss of carbonation in those few seconds between removing the cane from the beer and capping it? Even if you cap on foam, I feel like you're creating a small airspace at the top of the bottle. Won't the beer equalize pressure with this airspace? Or is it so small it doesn't matter?

Normal carbonation is around 2 volumes, right? (give or take). In a 22 oz bottle, that's 44 oz of CO2, (at STP, but stop splitting my hairs ). The equalization with 1 or 2 ounces of headspace is minimal.

Edit, in a 12 oz bottle, it's 24 oz CO2...assuming 1 oz headspace, that's 4% unfilled....assuming your beer equalizes with the headspace, you lose, what, at most 8%? (I'm going off of me carbonating beer at around 12 psi, which is about 0.8 atm, and since PV=nRT, PV is inversely proportional, so pressure doubles, volume halves, half the volume means you fit twice as much, so at a rough estimate double the % unfilled).

You really wanna be safe, I guess overcarb your beer by 4-8%?

BM's CPBF works just fine. That's why so many people have done it. I do mine a little bit differently because I too was worried about a loss in carbonation. Google "Poor Man's Counter Pressure Bottle Filler" if you want to see it.

You do need to cap fast. It's not like you can set aside six bottles and cap them after you've filled all six. You need to do them one at a time. There is a cushion of CO2 on top of the beer. If you get the cap on fast, you won't lose much CO2. I've had some of my bottles in the fridge for 6-8 months before I finally got around to drinking them and they still had great carbonation.