Many of us like things in greater quantities. More money, more expensive stuff, more horse power, more guns, etc, etc. Same tendencies are applied to muzzle speed. It is not necessarily a bad thing. But if you go too far with fps obsession, you might find yourself in a pitfall. Myth is faster fps will get you more range and accuracy. It's not always so. I am trying to establish a way to avoid the pitfall, and help you with increasing your replica's accuracy and range.
Feet per second (fps) describes how fast an airsoft replica shoots a projectile. Since 0.2g are the most easily found, measuring with 0.2g BBs has become a non-official standard. It is a useful measurement to compare two replicas. If both rifles shoot 0.2g BBs, and one shoots at 300fps and the other shoots at 400fps, you know which rifle is stronger. You can also measure how much your replica has been upgraded. For example, going from 280fps to 400fps is a vast improvement.
However, fps change if you change your ammo. If two snipers say that their rifles shoot at 400fps, but one is talking about firing with 0.2g, and the other one is talking about firing with 0.43g, the second rifle is twice as strong. Which is why it's better to use how much power your rifle or gun has with FORCE, just as it's better to use horsepower than how fast an automobile can go. Joule is universal measurement of force, and Joule does not change no matter what BBs you use. 0.2g fired at 400fps = 1.5 Joule = 0.25g fired at 360fps, etc, etc.
[Faster Muzzle Speed]
Faster muzzle speed will get you better accuracy and longer range to certain degree. If you are comparing two rifles, that is true. You have to know how to utilize that muzzle speed to get accuracy and range. With faster muzzle speed alone, all you get is more air resistance. Many would ignore air resistance, because isn't very noticable. However, air resistance is the #1 enemy for your rifle's accuracy and range. If you pedal on your bicycle as hard as you can on a flat road, you could get to about 35 miles per hour. If a tractor trailer is in front of you, slowly increasing speed as you increase speed, you will find that you can go as fast as 70 miles per hour. Wind resistance is what's stopping you from going that fast. Conversely, if your speed goes up, your wind resistance goes up tremendously. If you shot at 120fps, and now shoots 360fps, your BB is experiencing 9 times the air resistance. Unlike an automobile, a BB has no engine. Once it's fired, the only way for the speed to go is to go down. So, what's the result? As soon as you shoot a BB with faster speed, you lose almost all the extra speed very quickly. That's why auto makers worry about wind resistance, and some real steel bullets come with pointed tip and sailboat shaped tails to reduce the drag.
6mm BBs are tiny. And its shape cannot be changed from a spherical shape, because of the nature of hop-up. Because it's small, air resistance seems negligible. But it is the single greatest force working against a BB's flight. Faster muzzle speed, will get you far more air resistance. As a result, your replica will lose that faster speed almost right away.
We now have a dilemma. Faster speed seems like a good thing, when comparing two rifles. Also faster speed seems like a good thing again, when comparing upgrades. But as a rifle shoots faster, there is more air resistance. If you upgraded your gun to shoot 50fps faster, but because it's faster, it loses 50fps in a dozen feet. What was the point of upgrading it then? Don't worry, there is a solution.
[Name of the game: Keeping the energy]
In order to increase accuracy and range, your BB has to hold on to the energy given, all the way to the target. How do we keep the energy as long as possible? A shot's energy is expressed in two ways. Mass and velocity [weight and speed]. If you have less mass, velocity goes up. If mass goes up, velocity goes down. Let's say that your rifle shoots 0.2g at 400fps (1.5 Joule). If you use 0.25g in the same rifle, muzzle speed will go down to 350fps. If you are a blind follower of higher muzzle velocity, you will go for 0.2g with 400fps. That could be a bad choice.
Higher fps at the muzzle would be lost almost immediately due to air resistance. On top of that, you got that higher speed by shooting lighter BBs. Lighter BBs have less momentum (inertia). It loses speed faster still. As a result speed advantage is immidiately lost in about 20 feet.
If you had chosen 0.25g instead, you have chosen more mass and less speed. By choosing more mass, you have more momentum. And by choosing less speed, you have less wind resistance. It's a win-win situation, because you kept the energy in the form of mass. Mass cannot be lost, but speed can be lost very easily. What's the solution to the air resistance problem that's caused by fasfter fps? Use heaviest possible BBs for your replica. That will slow the muzzle speed down, which means you lose less energy to the wind. And heavier projectiles have greater momentum (inertia). Common sense seems to be telling you that lighter BB will go faster and further. But because of the stronger wind resistance and lack of momentum, lighter BB loses speed and momentum, right away. In about 100 feet, heavier BBs will end up going faster than lighter BBs, and eventually go farther too. In other words, if you upgrade your rifle, and don't upgrade your BB to heavier ones, your upgrade is almost useless.
[What to use]
As a result of less wind resistance, and more momentum, heavier projectiles go farther, and stay truer to their courses. Which is why snipers choose heavier BBs. If you are not a sniper, what do you use? That depends on your style of engagement. If you are a close quarter fighter, going room to room, you don't need accuracy. You simply need to spray as many BBs as possible. 0.2g would do just fine. If your target is beyond 30 feet, using 0.25g BBs would give you more accuracy and range. Of course, given two rifles with different speeds, it's better to take the rifle with higher fps. But given the same rifle, it's better to use heavier and slower BBs. This means, whatever you have, use as heavy BB as your weapon can handle. What's too heavy? That's your decision.
Because 0.2g seems to zoom out, and you can see 0.25g floating out slower, you get the impression that 0.25g is too slow and it does not go far. But that's only from muzzle to about 20 feet. From then on, 0.25g works better.
--<Close Quarter Combat>
Uzi, G3 SAS, Sig 552, Mp7, Mp5, G36, P90, Spetsnaz, and Thompson are all short to medium range weapons. These are mostly used as close quarter or recon weapons. If the range is within 30 feet, it's close enough that accuracy will not matter. In that case 0.2g would work just fine. But even these CQC weapons have effective range of about 100-130 feet. At this range, they could benefit from accuracy. Stock AEG is strong enough to handle 0.25g at this range. Adjusting scope or sight to about 60-70% of maximum range is recommended. For example, if maximum effective range is about 120 feet, adjusting scope or sight to about 80-90 feet would be desirable. That way, if you had to shoot a target at 100 feet, you can easily adjust by aiming a bit higher.
--<Med-Long Range Weapon>
M4 and AK family assault rifles would be medium range weapons, FAMAS, M14, M16, Type 89, Aug, and G3 variants would be longer range weapons. Routinely, these would engage at around 60-100 feet or even as far as 200 feet. No matter what their respective ranges would be, these could easily benefit from 0.25g BBs. Upgraded AEGs can handle even heavier BBs, such as 0.28g BBs.
BBs flying at longer ranges are exposed to more air. If it flies over heated surfaces, such as heated road, roof, or rocks, air rises. If it passes by a tree, it is affected by the air currents curving around the tree trunk. Corners of a building or any standing structure would have wind tunnel effect. If there is enough wind to move leaves that are heavier than BBs, there is a chance that wind would affect BBs. Heavier BBs are affected less by cross winds. 3-4 miles of cross wind would blow 0.25g BBs off-course by about a foot in 100 feet. 0.36g would move only about 4 inches. This is why snipers use heavier BBs.
Medium or long range weapons can certainly benefit from using heavier BBs. Stock AEGs can easily use 0.25g. Tokyo Marui recommends their 0.3g on stock VSR and stock PSG-1, but it may curve down too much to be effective beyond 100 feet. Personally, I think 0.3g on stock AEG is a bit too heavy, because it sacrifices too much range for the sake of accuracy. If AEGs are upgraded to 1.15 Joule (350fps w/0.2g), 0.28g would work. If AEGs can do 1.5 Joule (400fps w/0.2g), 0.3g can be effectively used.
It's not too difficult to upgrade a sniper rifle to shoot at 1.5 joules (400fps measured w/0.2g). I feel 0.3g is the minimum weight for sniper rifles that can shoot harder than 1.5 joules. If it's above 1.9 joules, 0.33g can be used. Above 2.3 joules, 0.36g can be used. The heavier they are the more accurate they will be. These numbers are not cut in stone. A 1.5 joule rifle CAN fire 0.33g, it's just a personal preference.
All BBs arch down eventually, but that's no reason to not use heavier BBs. No BB nor any real steel bullet will fly straight, but with heavier BBs, you can notice the drop more. Adjust scope to about 150 feet, or 180 feet. You will see BB going up and come down, just like the real steel. Aim a little low for targets within that distance, aim a bit higher for targets beyond that distance, just like real steel snipers would.
If you are interested, here is the data Battle Priest posted 2 years ago concerning some of the physics: http://www.airsoftforum.com/board/Muzzle-Velocity-Bb-Weigh-t20837.html
A stock AEG firing 0.2g at 279fps, will fire 0.25g at around 249fps. Lighter 0.2g BBs come out faster, heavier 0.25g BBs come out slower. After 60 feet, 0.2g would be going at 170fps, 0.25g would be going at 174fps. 0.25g is going faster at 60 feet. A heavier BB going faster means that more energy kept. Which means it will better stay on course. Lighter 0.2g lost 109fps, heavier 0.25g lost only 75fps. All the speed advantage 0.2g had is lost in 0.3 second. Did 0.2g get to the 60 feet mark faster than 0.25g? Not really. 0.25g got to the 60 feet mark in 0.3 second too.
Then where did 0.2g lose all that speed? It's right at the muzzle. 0.2g came out at 279fps. In 20 feet, it will lose 50fps and come down to 229fps. 0.25g came out at 249fps, but in 20 feet, it will lose 32fps and down to 217fps. 229 vs 217. 0.25g is a bit slow at 20feet, but 0.25g has more energy than 0.2g at this point already, because it's heavier. At around 40 feet, 0.2g is flying at 195fps, and 0.25g is flying at 193fps. Just about the same.
Which one goes farther? After 3 seconds, 0.2g is at 260 feet mark, and down to 41fps. After 3 seconds, 0.25g is at 284 feet, down to 49fps. 0.25g already has gone 24 feet farther, and still at higher speed.
Higher speed is immediately lost, right out of the muzzle. Because of this, it's better to use slower and heavier projectiles. Myth is that lighter BBs will go faster and further. But as you can see on the data, heavier and slower BBs will become faster and will go farther.
This article is about how one should use heavier BBs, if one wants better accuracy and range. As such, I was recommending heavier weight BBs for rifles of different strengths. I came to the problem of describing how strong a rifle is. I am recommending using heavier projectiles, and because of the change in mass, fps change also, so how do I describe how strong a rifle is? As mentioned at the beginning, Joules is a better measurment. Joules is horsepower for airsoft rifles. Instead of using fps, stick with airsoft horsepower, Joules.
Here is a very nice chart Battle Priest has provided: