Re: 240 Gibbs
Its basicaly a 6mm-06 improved with the shoulder bumped forward.
I personaly don't think that the Gibbs will offer any real benifit over the 6mm-06 Ackley improved as the regular 6mm-06 is pretty baddly overbore already. I don't think that the extra hassel of the fireforming the Gibbs case offers any real benifit in this bore diameter.
If you looking for a hot 6mm then I would go with the 6mm-06 AI , I say the Ackley version simply for the extended brass life that this desgine offers not for any performance benifits
Si Vis Pacem Parabellum