How do I change the integer?

This is the part of a gun script made by XAXA that I'm editing so that whenever a StringValue named "Magazine[5.56x45]" is retrieved from a storage box and relocated into a player's backpack the script will essentially utilize the "findFirstChild" function and if the said stringvalue is found I want it to add "1" to the "MagazinesCarried".

MagazinesCarried = 1 --There is no "StoredAmmo" or "Ammo" Numbervalues within the tool,  if the StringValue is found then I want it to add 1 to the current value.

--After the mag is retrieved nothing happens and no errors presented--
function ammAdd()
	local mag = tool.Backpack:findFirstChild("Stanag30") 
	if mag ~= nil then
	local ammo = MagazinesCarried + 1
	script.Parent.Parent.Stanag30:remove()	
	end
end

function updateHUD() -- INDEX: F-UPDTH ... Updates the HUD whenever the gun is reloaded or equipped. Uses F-UPDTA to update ammo.
	Hud.ModeLabel.Text = (fireModeValues[fireMode])
	if usesClips then
		Hud.MagazinesCarriedLabel.Text = "x "..MagazinesCarried
	else
		Hud.MagazinesCarriedLabel.Visible = true
		Hud.ClipPic.Visible = true
	end
	Hud.AmmoMaxLabel.Text = tostring(AmmoMax)
	updateAmmo()
end