The owner of a registered NFA firearm can allow another person to fire it as long as the owner (or if the owner is a corporation, a representative of the corporation) is present. That applies whether the firearm is loaned (to a friend) or rented. The point is that there is no transfer of ownership. If the person borrowing or renting the gun removes it from the purview of the owner (e.g., takes it home), that would be considered a transfer of control, amounting to an illegal transfer of ownership.
Jim
Jim