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These cartridges are filled with 8 grams of natural carbon dioxide. They have many uses from carbonating to pressurising.
These cartridges are widely used for pressurising/injecting Co2 into standard 2-inch neck plastic homebrew beer barrels. Check your barrel has the right adapter for these small CO2 cartridges. Keeping your barrel pressurised as it empties will ensure air is kept out and your beer is properly preserved.