well, he might be using a CO2 charger on the trip, but surely he's got a larger cylinder for at-home dispensing.
but assuming he doesn't and that's why he's priming and not force carbing in teh first place...yes, you can vent the keg, add more boiled & cooled sugar water, and put the lid back on.
if its only a little under carb'd you could shoot a full 12g CO2 bulb in there and let it soak that up and hope for the best.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
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