A third of you're description is wrong, it cannot fire common magazines. I checked. It only fires the ammo that comes with it.

Hi,

Are you using any mods? Which Factorio version are you running?
It doesn't come with an ammo, that comes separately.

Thanks

Hi again,

Double checked it with the latest version and it does work with both the firearm magazine and the piercing rounds magazine. You might be using some exotic mods or game version that might cause the problem. If you can provide the list of mods you are using alongside with your game version, I might be able to fix it for you.

Thanks

Oh really? yes I am using a lot of mods but wouldn't... let me just check the mod folder '145 items' oh... sorry I don't have time to type out the names of all of these, so I'll just take your word for it.

God! I love it when people complain, yet are unwilling to do anything about it when help is offered.

open CMD and type "dir /w > modslist.txt" from your MOD directory,
then simply copy n paste the text from modslist.txt

What is cmd? Is it console command related? I don't know how to use that but I found another way:

base
hacked-splitters
no-more-redundancies
Reactors
Research_Savr
VehiclesEquipement
what-is-it-used-for
AddAssemblerBatteries
Additional-Turret
Advanced-Electric
air-filtering
Alchemy
Algaculture
AmmoBox
Arborium
AussyBush
auto-research
autorecyclingplant
AutoTrash
Bergius_Process
better-storage-tanks
BigBelt
BigDrill
Big_Brother
BioTech
Bio_Industries
Biter_Seasons
blueprint-book-2
Bottleneck
BurnerLeech
Crafting_Speed_Research
deforest-planner
Diesel-Fuel
DoubleFurnace
DrillingPumpjack
easy_refineries
electric-vehicles-lib
Expanded_Robot_Tech
ExplorationVehicle
factorio-reach
Factorio Directed by Michael Bay
FactorioExtended-Core
Factorissimo
FastLongInserters
fish-sandwich
flamenades
GDIW
HandyHands
InterfaceChest
ItemCount
landfill-blueprints
LateUpgrades
launch-control
Lejv-armor
loader-for-game
LogisticsMining
MeteAutoVehicleRepair
MeteMinigun
MetePlasmaClip
MetePlasmaDefense
MeteRocketry
MeteVehicles
military-extended
Military_Research_5
MiniBasicStart
MK2-Mod
momining
more-bullet-upgrades
more_oil-production
moskin
motrans
moweather
NapusMod
NaturalTreeExpansion
NPUtils
Nucular
OceanLiners
Omnibarrels
Orbital Ion Cannon
original-music-hd
Orphan Finder
PeppeBotStart
persistent-research
Pipe-Manager
Plasma Shotgun Shells
Power Armor MK3
PreciousPreciousCaffeine
Rampant
Random
ReactorsNucularFix
Resource-Conversions
Robo-Charge-1x1
RoboCharge
robotMiningSite
Robot_Cleanup
SenpaisElectricTrains
ShuttleTrain
solaire-lights
Spitter Delay
Squeak Through
StoneWaterWell
Straight_Pipes
SuperTank
sustainable-energy
tankwerkz
Teleportation
tiredhands
Tmod
Treefarm-Lite
tree_collision
VersepellesAlienFarm
VersepellesAquaFarm
VersepellesDeepQuarry
Warehousing
Waterfill
wireless-charging-lib
Zoom
Beyond
electric-vehicles
FactorioExtended-Equipment
FactorioExtended-Logistics
FactorioExtended-Machines
FactorioExtended-Power
FactorioExtended-Transport
FactorioExtended-Weaponry
GDIW_Bergius
MiningTools
moindustry
mopower
SevenIndustries
wireless-charging
FactorioExtended-Storage
mocombat
(copied and edited from mod-list.json)

And, now that I've gone and looked,
I notice that military extended is in the description of both the minigun and the heavy duty bullet magazine (which is all it excepts) so that's probably the cause, as mete said.(edit: just checked, it was)