With proper shot placement from the proper angle with the proper projectile the effective reliable killing range of a 7-30 Waters is 250yds, or however far you can keep EVERY shot from ANY rest that you wil be using on a 8" circle. Can it kill further than 250yds? If you're an excellent shot and very lucky, but I wouldn't rely on it much past 200yds. You need perfect conditions and be a VERY good pistol shot to hit a simple paper plate at 250yds off of a field rest--EVERY TIME. That 250yds depends very heavily, as GB stated, on proper projectile selection. I've had good luck with the 130gr Sierra SSP bullet, and the Hornady 120gr SSP bullets out to about 200yds with the 7mm TC/U and 7mm BR, but you have to have a broadside shot in my opinion, and you'd best not be hitting the deer solid on the shoulder at that range. I shoot a .284 Winchester in a XP-100 now, and I can't find any field situation where I would pull the trigger on an animal past 250yds with a superb rest, but I would have the confidence to know that the cartridge has plenty power and the bullets will perform at that range.
The bottom line is that no cartridge is going to reiliably kill anything any further away than you can hit it with a bullet of proper design in the proper location. The effective range is up to you and your proficiency and hopefully sound judgement as a hunter.