local function on_research_finished(event)
    local research = event.research
    if string.sub(research.name, 0, 26) ~= 'physical-projectile-damage' then
        return
    end
    local multiplier = 4
    if global.shotgun_shell_damage_research_multiplier then
        multiplier = global.shotgun_shell_damage_research_multiplier
    end

    local modifier = game.forces[research.force.name].get_ammo_damage_modifier('shotgun-shell')

    modifier = modifier - research.effects[3].modifier
    modifier = modifier + research.effects[3].modifier * multiplier

    game.forces[research.force.name].set_ammo_damage_modifier('shotgun-shell', modifier)
end

local event = require 'utils.event'
event.add(defines.events.on_research_finished, on_research_finished)