Zulu Hit it right on.. effective range is what you are after.
and his Rule of Thumb (9/10 on torso Target @ Range) is Dead On.
In my mind you should always adjust the hop-up for a sniper rifle.
The stock Hop-up unit on an M-24 is touchy and can easily over Hop...
Use of a heavier weight BB is recommended (.28g+ ... I use .40g)
My MAX Effective Range on my M-24 is 220ft .. I wont even attempt a shot over 240ft.. to me its just not worth it. I mostly engage at ranges between 150ft - 200ft.
And to build an M-24 to meet an effective range of 200ft is not difficult.