i messed up with key down i just needed to replace keydown to button1down
m.KeyDown:connect(function()
if g.stats.ammo.Value >= 1 then
local a = g.gun.bullet:Clone()
a.Parent = game.Workspace
a.Anchored = false
a.Transparency = 0
g.gun.bolt.Transparency = 1
g.gun.bolt1.Transparency = 0
g.gun.shot.smoke.Enabled = true
g.gun.shot.flash.Enabled = true
g.stats.ammo.Value = g.stats.ammo.Value - 1
g.gun.Handle.FireSound:Play()
wait(0.01)
g.gun.bolt.Transparency = 0
g.gun.bolt1.Transparency = 1
g.gun.shot.smoke.Enabled = false
g.gun.shot.flash.Enabled = false
else
print("reload")
end
end)

2 comments
What is m? What is a? What is g? Noone will help you, if you simply copy-paste your script and say it's not working, so, please, provide a description of what you're trying to do, what isn't working as expected.
m is mouse , a is copied bullet , g is ignore model like arms etc.