Ivenhoe,
You posed an interesting question regarding cartridge shape and inherit accuracy. The fact is, case shape does play a role in accuracy potential, but only at the benchrest level. Faster powders and shorter burn paths increase inherit accuracy, i.e. a benchrest rifle chembered in .22 PPC may be capable of 0.10 MOA at 100 yards whereas the same rifle in .223 might only be capable of 0.12 MOA. There have been some really well done tests to confirm this, but as you can see, it doesn't make for squat difference in a hunting rifle. So don't worry about it
A .257 Roberts or 7x57 Mauser is as good as it gets for deer at reasonable ranges. Although I prefer the way that the .257 Ackly Improved Roberts looks---I'm a sucker for sharp shoulders