Hop up "off" is a relative term, depending on the rifle. Most guns still provide a modicum of hop-up at the lowest setting.
150 feet is an estimated "effective" range (I'm sure Mojo knows what he's talking about, I've never touched a CA M24). MAX range on an airsoft rifle at 340 fps could be 500 feet, but that's if you pointed up with a .12g at a 45 degree angle. See what I'm saying?
Effective range is the range the rifle will still be accurate enough to hit a torso-sized target at with great consistency
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.
I had to run .28s minimum at 350 fps to get a straight and level flight and .43s when it was at 500fps.
I did do alot of mods to mine though like a TDC hop adjuster, plus a few little tweeks here and there just to make it more consistent. Mostly cuz I was to tight to buy a PDI chamber at the time, but from what I've read since they're a pain in the jaxy anyway so I'm glad I stuck with the stock unit and my mods up to the point I sold it.