THANK YOU COLDFIRE for knowing Eugene Stoner and Kalishnikov! I was almost depressed when I was reading that "nobody knows who they are" and "they don't get credit". That's like saying that Henry Ford had nothing to do with Ford Motor Company (or the assembly line for that matter)!
Also, about the calibers, since you are inventing a gun, you can invent the caliber you want. Something to think about is ballistics in general. There is typically a trade off between stopping power and trajectory, especially when you are worrying about recoil. I kind of like the idea of the 6.8 compromise, and in fact, there are some very powerfull and accurate cartridges around the 6.5 mm caliber.
Think of three things when thinking of a cartridge: Velocity, Energy, and Trajectory. Energy is directly affected by Velocity, and Trajectory is definately affected by Velocity. Also, the ballistic coefficient is important, but that matters little in this case. The more powder you have, the faster the bullet, thus more energy, and flatter trajectory. Furthermore, the bigger the bullet, the SLOWER it goes, but with more energy.
Probably the best thing to think about in this case is wildcats, which are "homemade" cartridges using a large bullet casing necked down to a different bullet caliber than normal. For instance, take a .250 savage and "neck it down" to a .243. That idea for a 6.8 mm round is good. You could (in real life of coarse) play around with cartridge sized and powder charges to get a good, solid, high energy round with a flat, accurate trajectory. BRILLIANT!!
Sorry to ramble on, I love talking about guns and reloading and stuff. My wife won't hold those conversations with me, and my friends now live in a different town, so that leaves this forum! THANKS!