Larry,
If you pop on over to Geoffroy's website, you will see he has photos of a 24 gauge R10 Darne that was produced with 70mm chambers.
I've seen 70mm marked 24 gauge ammunintion in Europe-all I can tell you, is it wasn't Winchester or Remington. I also have some Spanish ammunition that is marked 70mm, but measures 67mm after being fired, and more like 65mm as loaded.
I have found 20 gauge to be the hardest to get really good patterns out of, and can't imagine 24 gauge is much of an improvement.
Best,
Ted
