@DigitalHuman
You can do that in the weapon '.meta' files. Search them for 'FlashFx' & you'll find various values to edit:

Example:

<Fx> <EffectGroup>WEAPON_EFFECT_GROUP_SHOTGUN</EffectGroup> <FlashFx>muz_shotgun</FlashFx> <FlashFxAlt /> <MuzzleSmokeFx>muz_smoking_barrel_shotgun</MuzzleSmokeFx> <MuzzleSmokeFxFP>muz_smoking_barrel_fp</MuzzleSmokeFxFP> <MuzzleSmokeFxMinLevel value="0.300000" /> <MuzzleSmokeFxIncPerShot value="0.100000" /> <MuzzleSmokeFxDecPerSec value="0.200000" /> <ShellFx>eject_shotgun</ShellFx> <ShellFxFP>eject_shotgun_fp</ShellFxFP> <TracerFx>bullet_shotgun_tracer</TracerFx> <PedDamageHash>ShotgunLarge</PedDamageHash> <TracerFxChanceSP value="1.000000" /> <TracerFxChanceMP value="0.750000" /> <FlashFxChanceSP value="1.000000" /> <!-- try set to 0.000000 --> <FlashFxChanceMP value="1.000000" /> <FlashFxAltChance value="0.000000" /> <FlashFxScale value="1.000000" /> <!-- try set to 0.000000 --> <FlashFxLightEnabled value="true" /> <!-- flash of light on environment, usually off, but make sure set to false --> <FlashFxLightCastsShadows value="false" /> <FlashFxLightOffsetDist value="0.000000" /> <FlashFxLightRGBAMin x="255.000000" y="93.000000" z="25.000000" /> <FlashFxLightRGBAMax x="255.000000" y="100.000000" z="50.000000" /> <FlashFxLightIntensityMinMax x="1.000000" y="2.000000" /> <FlashFxLightRangeMinMax x="2.00000" y="2.500000" /> <FlashFxLightFalloffMinMax x="1024.000000" y="1536.000000" />

Most original non-dlc weapons (including some vehicle weapons, Lazer, Buzzard, Hunter, Rhino etc) are in a 'weapons.meta' here:

...\mods\update\update.rpf\common\data\ai\weapons.meta

There are also other dlc & such weapon data files dotted about the game (use OpenIV's Ctrl+F3 Search to find them by name):

Standard Handheld Weapon Files:
weaponautoshotgun.meta
weaponbattleaxe.meta
weaponbottle.meta
weaponbullpuprifle.meta
weaponcombatpdw.meta
weaponcompactlauncher.meta
weaponcompactrifle.meta
weaponcougar.meta
weapondagger.meta
weapondbshotgun.meta
weaponfirework.meta
weaponflaregun.meta
weaponflashlight.meta
weapongarbagebag.meta
weapongusenberg.meta
weaponhandcuffs.meta
weaponhatchet.meta
weaponheavypistol.meta
weaponheavyshotgun.meta
weaponhominglauncher.meta
weaponknuckle.meta
weaponmachete.meta
weaponmachinepistol.meta
weaponmarksmanpistol.meta
weaponmarksmanrifle.meta
weaponminismg.meta
weaponmusket.meta
weaponpipebomb.meta
weaponpoolcue.meta
weaponproxmine.meta
weaponrailgun.meta
weaponrevolver.meta
weapons.meta
weaponsnowball.meta
weaponsnspistol.meta
weaponspecialcarbine.meta
weaponstonehatchet.meta
weaponswitchblade.meta
weapons_arena.meta
weapons_assaultrifle_mk2.meta
weapons_bullpuprifle_mk2.meta
weapons_carbinerifle_mk2.meta
weapons_combatmg_mk2.meta
weapons_doubleaction.meta
weapons_heavysniper_mk2.meta
weapons_marksmanrifle_mk2.meta
weapons_pistol_mk2.meta
weapons_pumpshotgun_mk2.meta
weapons_revolver_mk2.meta
weapons_smg_mk2.meta
weapons_snspistol_mk2.meta
weapons_spacerangers.meta
weapons_specialcarbine_mk2.meta
weaponvintagepistol.meta
weaponwrench.meta
weapon_ceramicpistol.meta
weapon_hazardcan.meta
weapon_navyrevolver.meta
weapon_tranquilizer.meta

Vehicle Based/Other Weapons:
vehicleweapons.meta
vehicleweapons_akula.meta
vehicleweapons_apc.meta
vehicleweapons_avenger.meta
vehicleweapons_barrage.meta
vehicleweapons_bombs.meta
vehicleweapons_bombushka.meta
vehicleweapons_bruiser.meta
vehicleweapons_caracara.meta
vehicleweapons_chernobog.meta
vehicleweapons_deathbike.meta
vehicleweapons_deluxo.meta
vehicleweapons_dogfight.meta
vehicleweapons_dune3.meta
vehicleweapons_flame.meta
vehicleweapons_hacker.meta
vehicleweapons_halftrack.meta
vehicleweapons_havok.meta
vehicleweapons_hunter.meta
vehicleweapons_issi4.meta
vehicleweapons_khanjali.meta
vehicleweapons_menacer.meta
vehicleweapons_microlight.meta
vehicleweapons_mine.meta
vehicleweapons_mines.meta
vehicleweapons_mines_rc.meta
vehicleweapons_mogul.meta
vehicleweapons_monster3.meta
vehicleweapons_mortar.meta
vehicleweapons_mule4.meta
vehicleweapons_oppressor.meta
vehicleweapons_oppressor2.meta
vehicleweapons_paragon2.meta
vehicleweapons_pounder2.meta
vehicleweapons_rogue.meta
vehicleweapons_scarab.meta
vehicleweapons_scramjet.meta
vehicleweapons_seabreeze.meta
vehicleweapons_speedo4.meta
vehicleweapons_strikeforce.meta
vehicleweapons_stromberg.meta
vehicleweapons_tampa3.meta
vehicleweapons_thruster.meta
vehicleweapons_trailer.meta
vehicleweapons_tula.meta
vehicleweapons_vigilante.meta
vehicleweapons_volatol.meta
vehicleweapons_xmas17.meta
vehicleweapons_xmas18.meta
vehicleweapon_jb700.meta
vehicleweapon_rctank.meta
weaponcannonblazer.meta
weaponturretboxville.meta
weaponturretinsurgent.meta
weaponturretlimo.meta
weaponturrettechnical.meta
weaponturretvalkyrie.meta
weaponvehicleruiner.meta
weaponvehiclesavage.meta
weaponairdefencegun.meta