ðeɪ ɐkwˈaɪɚd ðɪs nˈɪkneɪm bɪkˈʌz ðə ɹˈɛdʒɪmənt wʌzðə fˈɜːst ɪn ɐmˈɛɹɪkən hˈɪstɚɹi tə hæv ɐn əfˈɪʃəl ˈɪʃuː ɹˈaɪfəl ˈɛm wˈʌn θˈaʊzənd ˈeɪthˈʌndɹɪd fˈoːɹɾi wˈʌn mˌɪsɪsˈɪpi ɹˈaɪfəl ɪnstˈɛd əvə smˈuːðboːɹ mˈʌskɪt.
They acquired this nickname because the regiment was the first in American history to have an official issue rifle (M1841 Mississippi rifle) instead of a smoothbore musket.