Not many people want to mess with neck turning. And more recently no-turn chambers have done well in competition, so more people are deciding not to neck turn. Several pre-fit barrels do have necks that are not SAAMI spec. Most 6BR have .272 no turn necks. But those really thin custom neck diameter barrels don't sell. Stocking different neck diameters would end up in a bigger inventory that didn't move as quickly. This is because if you are wanting to get really competitive you want to get s blank and either chamber it yourself or have your favorite gunsmith chamber/thread it for you.