Because I always loved the Weapon Guides from GDWs days, I've picked up a few of the 3rd party weapons guides published on DTRPG under the FL license. I think the guides by Geir Ove Thorsveen are the best and most consistently written ones. He has added some nice optional rules covering ROF and bracing weapons, as well as concealability and other gaps in the FL rules.
His GPMG / HMG supplement has the M134 Minigun with an ROF of 10 and different barrel options for the KPV machinegun (ZPU-1, ZPU-2, ZPU-4). Basically, if the KPV has an ROF 4, the ZPU-2 (2 barrel) is ROF 7, the 4 barrel is ROF 10. Not a perfect solution, but pretty workable.
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https://www.drivethrurpg.com/product...-North-America)