M6P pocket pistol
From Halopedia, the Halo wiki
M6P pocket pistols can be loaded with four rounds of high-explosive (HE) ammunition.
In the first draft of Halo: Retribution, the novel's author, Troy Denning, only called the weapon a "pocket pistol". The designation "M6P" was later added to tie the weapon to the M6 series, at the suggestion of 343 Industries.
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