Borrowing this from airsoftforums.com. Not my work!
first hit in a google search for hopup BB flight path chart:
What Is Hop Up?
For those of you who do not know what hop up is in the first place, it is the backspin put on a bb as it brushes by a hop up nub near the top of the barrel. The nub causes friction which makes the bottom of the bb slip out faster than the top to create a predictable spin. This all works with the Bernoulli principle to case a lift on the bb that can negate gravity for a longer amount of time giving your bb more range and a straighter path. Here is a simple diagram provided by airsplat of how it works:
How Does It Affect You?
Now that we understand how hop up works and the function of hop up we will discuss the issues that occur, particularly with snipers.
Theoretically hop up should have absolutely no effect on accuracy other than vertical accuracy. Since the hop up is only supposed to create backspin, the bb trajectory should not be effected left or right. I say theoretically because in an ideal world we would all hold our guns perfectly level, and if you want to get that perfect shot every time you will want to make this ideal world a reality.
The problem really is that the hop up nub is at a fixed point while your position is not.
When you hold your rifle perfectly level the results should follow a straight trajectory:
If you were to, let's say, tilt your rifle to the right on your shot the results would follow a trajectory curving to the right:
These conditions are magnified on snipers as their bbs spend even more time in the air. The further the shot the more hop up, especially right or left spin, will affect the trajectory. Whereas hop up on a level shot will simply stop its resistance and begin to drop, gravity will not negate the effects of an non-level shot but simply direct them at a more downward angle.
Originally Posted by (silent_soul @ Jul 9 2009, 11:10 AM)
what im saying is a bbs flight path that is straight is easier to use than when that bbs path arcs up and back down. with that said if you can get a gun to shoot almost completely straight with almost no arc(thus straight with good range) it wont matter if the gun is perfectly level.
I have never found a use and probably wont for a level but thats thanks to the days of shooting with RS and airsoft ive done.
I understand what your saying but it seems more theory and a good idea than actual proven method. and I only say this as I have never set up a gun to shoot with a large arc or any unless I had no other choice. with that said I aim to get my guns shooting as flat as possible thus a level is unneeded. plus I rarely shoot past 200 feet so a level shooting gun out to 200 feet is nothing impressive.
Okay I understand what you are trying to say now and to an extent I agree. The only issue is that when your hop-up is adjusted to get your bb to fly in a straight path as you were talking about it means that the bbs spin is canceling out the effect of gravity for a given distance. That means that the lift is equal to the force of gravity. When the gun is not level then the spin is now creating lift in a different area of the bb. What used to be equal to the force of gravity in all directions is now unequal as the lift is now in a different position which requires less lift than given by what creates a straight shot on a level plane. I will explain it with pictures and Trigonometry/physics.
Okay if your bb is flying perfectly straight as you have said this would be the exact lift, that is, the force of lift on the bb would be equal to the 9.8m/s^2 force of gravity and the bb would not fall at all. We also assume there is no force or equal force horizontally on the bb as we said it flies straight.
Now let's say you shot on a 45 degree angle just to make the math easier. Given this angle and assuming we are using the exact same hop-up with the exact same result of spin (ie the force in both examples is the same) then with some trigonometry we can calculate that the force of the bb is changed from the previous shot. In fact, now the bb is falling with a force of 1.46m/s^2 and is moving to the right with a force of 8.34m/s^2.
So How Can I Fix It?
The solutions are few but all are relatively simple.
One method would be good old fashion practice. You simply need to learn what it feels like when it is level and be able to maintain this while under the stress of battle.
Some people have posted about epoxying a small level to their rifle that they can quickly glance at before taking a shot. The level could be placed anywhere you chose on your rifle but I would suggest placing it somewhere it cannot catch light as they are often a clear plastic and have a liquid, both of which can cause a reflection that could give you away.
You can also find scope levels at various places online with a Google search.
edit: images hosted on my pb account.