Sure there is. All coils have internal resistors - i.e. the windings. A coil sold as not needing an external ballast resistor for a 12 volt system simply has the equivalent of the the external ballast resistor windings rolled into the internals.
Outside or inside - it's still a bunch of small wire windings to create resistance.
Regardless if a "6 volt" coil with separate resistor outside, or a "12 volt coil" with no outside coil of wire or carbon resistor wire (like GM and Deere use sometimes) - it will all total somewhere around 3-4 ohms on the primary.
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