I have all the giibs between 240 and 338 except the 270. I like the 264 and 7mm better than the 270 so didn't do the 270. You are right about the neck differences. And take the advice of using 30-06 brass and necking down to form a good headspace off the shoulder. The gibbs will be faster than the 270 AI. I have both in all those calibers and in every case the gibbs is faster than the AI. Gibbs cartridges never really took off because of the short neck many don't like. Mine have done well even up to 338 caliber and the short neck has not been a concern with me. Basically all my gibbs are the virtual equivalent of the standard magnum counterparts. The 7mm gibbs same as the 7mm rem mag, 264 gibbs same as the 264 win mag, 338 gibbs same as the 338 win mag, 240 gibbs faster than 240 wby mag. 30 gibbs equals 300 wsm
. The gibbs will be slightly slower in those cases but they are so close you would not notice the difference in a hunting situation. I also have some calibers on the JRS design which uses the slightly longer 280 case and I like those because you can do the gibbs case capacity and it still leaves a longer neck.