sˈʌtʃ sˈɛkənd bˈɑːɾəlz ɑːɹ ˈɔfən θɹˈiː lˈiːɾɚz, ænd mˈeɪ biː mˈaʊntᵻd ɪn vˈɛɹiəs wˈeɪz fɹʌm tˈæŋk bɹˈækɪt tə slˈɪŋ mˈaʊntɪŋ.
Such second bottles are often 3 litres, and may be mounted in various ways from tank bracket to sling mounting.