There is no weapon reload event, so you can check if a player equiped a weapon with

IsPlayerArmed = GetSelectedPedWeapon(PlayerPedId()) ~= GetHashKey('WEAPON_UNARMED');

This will return true or false (BOOL) wheater player equiped a weapon or not. When the Player equiped a weapon then check if the player press the R Key:

IsControlJustReleased(0, 80)

See all controls here Controls - Cfx.re Docs

local IsPlayerArmed = GetSelectedPedWeapon(PlayerPedId()) ~= GetHashKey('WEAPON_UNARMED');

if IsPlayerArmed and IsControlJustReleased(0, 80) then
  -- Deactivate your Progressbar
end