My MP5(with a Beeman SS-3 on a claw mount) will consistently shoot 4-6" groups at 100 yards using Federal 115 grain 9BP's. At 15 yards it shoots ragged little holes on semi, and ragged little groups of holes on full.
Ammo does make a big difference in accuracy. I've had the best luck with Federal, and the worst with the old "China Sports" Norinco stuff. The Norico is fine for blasting, but would only do about 12-15" at 100 yards.
Using iron sights, and depending on the gun, I would think 100 yards is reaching the maximum realistic range, especially with an open bolt gun. I think 200 yards would be pushing it. Your going to get the best longer range accuracy from the closed bolt guns. I tend to agree with the 50 yards and under estimate for pistol caliber guns.
Full auto fire is a close range thing, if you want to be effective. 25 yards is about it, and 15 and under is better. Even with full auto only open bolt guns, its pretty easy to squeeze off single shots on demand once your familiar with the gun, and accuracy can be quite good. If you want to take a deliberate, longer range shot, you dont have to shoot in bursts, although it is difficult to do with the higher rate fire guns like the MAC's.