I can tell from this that you really are a .260 shooter who has no experience with the 6.5 Creedmoor. Nothing wrong with that, but....
Unless you order .260 brass off the Internet, 6.5 Creedmoor brass is far easier to come by--especially . Not only is easier to find in most gun stores, but you can normally buy bunches of once-fired stuff (usually Hornady, which is very good) on the Campfire Classifieds. This is because 6.5 Creedmoor Hornady ammo is so affordable and accurate that many people just shoot factory, and then sell the brass. And that affordable 6.5 CM ammo (which means brass) is widely available in sporting goods stores, and even Walmart.
Yeah, you can neck up.243 brass, but some .243 brass isn't very good, so you end up with not-very-good .260 brass. I learned this a long time ago after getting my first custom .260. Ended up buying Lapua .243 brass, because Lapua didn't make .260 brass yet. (I now shoot Lapua brass in my present .260, a Tikka T3 Superlite, along with a more recent brand of really good brass.)
Another thing I tried, which worked very well, way back in 2010 after purchasing my first 6.5 Creedmoor, was fire-forming .22-250 brass, just in case Creedmoor brass was difficult to find. This worked very well with the Cream of Wheat method, and I reported on it in an article for HANDLOADER magazine. But I never did it again, because very soon there was zero difficulty in finding 6.5 Creedmoor brass.
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