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