Case shape and capacity. From small to large, the Creedmoor uses a modified .30TC case which in turn is a shortened development of 308 Win / 260 Rem. It has less taper than '308 family cases' and 30-deg shoulders instead of 20, but ends up with a little less capacity because the case OAL has been reduced. (That allows a better bullet seating position within a 2.8-2.9" COAL for short-action magazine fed rifles.) The 6.5 Hornady PRC (Precision Rifle Cartridge) is a development of the fatter / higher-capacity Remington SAUM family of cases, and again has less internal capacity than its parent. Nevertheless, this will be a powerful high-velocity number given substantial case volume and powder charge weight increases over the 6.5X47mm Lapua / 6.5CM / 260 Rem / 6.5X55mm group of 6.5s. The SAUM is also a very strong case designed for 65,000 psi pressures. As with the parent, the PRC needs a magnum bolt-face, its case-head / rim having a larger diameter than the Mauser 7.9 / .30-06 / .308 design that the 260 and 6.5CM use. It also has a fatter body. 6.5-284 was originally a wildcatted development of the 1960s 284 Winchester that uses a rebated rim design (the aforementioned 0.473" nominal from the 7.9mm Mauser originally) but a fatter near magnum body above the rim and extractor groove. It was adopted by Norma Precision in 2000 and given factory status and regularised dimensions. In terms of case capacity as measured by the amount of water they hold in grains weight, the main players are: 6.5X47L ............. 48gn 6.5 CM .............. 53.5gn 260 Rem ............ 54-55gn 6.5-284 Norma ... 66gn 6.5 H. PRC .......... 69gn (estimate) and ........ for comparison, the 260 Nosler .......... 93.5gn Going back to your trio, the 6.5 H. PRC therefore has around 15gn higher water capacity than the Creedmoor, or ~28%. The rule of thumb is that all other things (pressures, bullet weight, barrel length) being equal, around a quarter of the % case capacity change is translated into % MV change. So the PRC should produce ~7% higher MVs than the CM, or in practical terms a typical 2,800 fps 140gn bullet load in the Creedmoor increases to ~3,000 fps. As the PRC will be rated at a higher pressure though, the case being designed for 65,000 psi, it should produce a bit more on top of that if loaded to its potential. http://bulletin.accurateshooter.com/2017/10/new-hornady-6-5-prc-precision-rifle-cartridge/ The 6.5-284 will fall in between the pair, but its potential performance will be closer to that of the PRC than the CM. The 6.5-284 has a reputation as a fine performer capable of superb precision, but offset by short barrel life for high round count disciplines like F-Class. The PRC will be worse in that sense, so it looks like it will be a superb long-range medium game round, and may see some benchrest competition experimentation, but is unlikely to see large-scale adoption in other competition disciplines.