The Pursuit of Happiness is a highly modified 5mm minigun.
The Pursuit of Happiness, or "The Poh," is as ordinary as any minigun can be. It's tri-barrels and gunner's sight hint to a former life as a mounted gun atop an armored vehicle and the shredder modifcations infer that a super mutant once got it's hand on it. It is not unseemly powerful but is a precious weapon to the Minutemen, as they normally deal in light laser weapons, and such an effective gun, to them, is praiseworthy.
The name is a joke- obviously two thousand rounds shot after you (in pursuit) per minute is not a happy thing.