I’m making this guide for several reasons, not least because I’m sad and have nothing better to do, or maybe I’m procrastinating from doing real work, but I’d also like to get down on paper some thoughts on the science of our pew pew guns and clear up some misconceptions or at least get a discussion going to better our understanding.
This guide is going to be a combination of actual science (well, as close as I can get to actual science ) and some personal experience, I’m by no means the most experienced airsofter out there but I’ve tried over the years to get an understanding of what’s going on and for those new to the hobby this might save some time barking up the wrong tree as I’ve done.
Part 1: the goal
Well this is simple, our goal is to shoot some random stranger on a farm on a Sunday and “kill” him, or at least hit him with a plastic pellet and for him to shout “hit” and plod off back to a chequered flag.
This sounds pretty easy, but we need to start somewhere, so what do we need to make this happen?
Well we need to hit him for a start, always helps, but not only that but we need to hit him hard. That’s not to say we need to hurt him, but whilst we can argue all day about wether or not people should wear plate carriers the short of it is you need your bb’s to hit with enough energy that he either feels it or hears the impact (which on a field of explosions and other people pew pewing can be difficult).
So when our bb hits the target it needs energy.
This is why I dislike to some extent the “you can get lots of range from a 328fps gun” argument, yes you can, but how far the bb travels isn’t the only part of getting a kill and just hitting isn't always enough sadly.
Part 2: some misconceptions
One thing that I see a lot in various threads is “upgrade your fps it’ll give you more range” or “change to x y or z hop it’ll give you more range” or “use x type of bb it’ll give you more range”, which is both right and wrong depending on how you judge range which will become more clear later.
The thing to remember is once your bb leaves your barrel it doesn’t care what happened before, it could have come from a speedsofter’s £3000 hpa or some kids £90 rental ak, it only has 3 things on it’s side to fight the wind/tree branches/gravity to get to the target and these are:
Backspin- this gives us our wonderful magnus force that will fight gravity and keep our bb in the air without needing to go at proper bullet speeds
Mass- how much the bb weighs
Energy- kinetic energy, we often like to use fps as our measurement as it’s how we can best record our performance but what really matters is energy
So we should probably cover these 3 as they apply to our bb in flight.
part 3- backspin
backspin gives us our magnus force, the science behind it is quite interesting and worth a read if you’re interested in expanding your knowledge but in reality the only thing that we need to know is if our hop unit is set right.
Now this is different for every setup, how fast the bb is actually spinning doesn’t really matter to us, it could be 1rpm or 10,000 rpm, we don’t care we just want our bb to fly straight.
Now personally I set my hop units for a little bit of rise, about 20cm, so that when the bb comes out it’ll rise up by 20cm then start falling back down. The logic here is that if I’m aiming for someone’s chest it’s not going to jump up over their head, miss then drop back down behind them. We’ve all been there where our hop has made us miss someone.
In terms of the physics the amount of lift we get depends on 2 things:
The speed of rotation, which is pretty much constant after the bb has left the barrel (well, it slows down a bit but not much)
The velocity of the bb
When we “set” our hop unit what we’re doing is setting the speed of rotation to get the right lift, but if we change ammo or change springs the bb will either be going slower (needing more spin to get the same lift) or faster (needing less spin).
Now the important thing to remember is that if we’re spinning x rpm, then there’s y velocity that gives us the exact amount of lift to fight gravity, but of course a bb doesn’t stay at y velocity, it slows down which is where mass comes in.
Part 4- mass
So we have our spin, but as I mentioned bb’s slow down, and this is the misconception I had many years ago that faster bb=more range
The important thing is air resistance, which the faster you go the more of it you have so your bb slows down quicker (moving it out of that sweet spot for the hop spin to work)
So say we have a 1j gun, we fire at 328fps on a .2g bb and 270fps for a .3gbb
well that .2g bb is going faster, so it has more drag, and it’s lighter so the drag has more effect. The end result is the bb slows down very quickly and it doesn’t take long for it to slow down so much our backspin stops lifting it.
Whereas the .3g bb is slower and harder to slow down, sure it might have started out slower but over time it keeps it’s velocity and hence stays in that nice range where the backspin can lift it.
So in theory at least a heavier bb spun sufficiently will for the same energy always go further, take a look at the spreadsheet at the bottom and see what I mean.
But wait, it gets better!
Remember I said we need 2 things to hit a target? Well a heavier bb gives us both!
In the real world we’re shooting through wind and worse leaves, these are forces just like regular ol’ drag and a heavy bb will be blown about less by the wind and has a better chance of punching straight through those pesky leaves and hitting the target, so we get more accuracy, yay!
But not only are they more accurate they also carry their energy better, so when we do hit the target at range it’s going to hit it harder, and thus make enough force and noise to let the target know they’ve been hit.
So where’s the downside? Why wouldn’t you run sniper .48’s in every gun you own and on every site? Well several reasons:
1. Cost, damn that pesky real world economics, but obviously heavier bb’s cost more. If you find somewhere selling .48’s for the same price as .2’s then let me know because that’s awesome. Those of us that own support guns with high rates of fire will most likely wince at having to fill it up with costly ammo only to be sprayed into the ether, likewise snipers who don’t shoot much can afford the good stuff hence why they like to run heavy.
2. Travel time, again this is the real world being all awkward and getting in the way of our lovely discussion but remember that our target may not be nice enough to stand still and wait to get hit, especially if he sees our bb’s coming towards him. Heavy bb’s might carry their energy better but they’re slower and take longer to go the same distance, not much longer, but it's enough.
3. Getting the lift, unfortunately the same properties that make a bb great in flight make them bad for spinning up in the hop unit, and this means that you may find you can’t run them without upgrading your gun and if (like me at the time) you don’t understand fully why it’s happening. This is the “more hop” argument for things like maple leaf/flathop/rhop guns, it isn’t about getting lots of hop on .2’s, it’s about being able to get enough hop on .3’s or heavier.
4. When you don’t need range, now I play mostly outdoors on sites limited to 1j, so for me getting every drop of performance I can is paramount, but say you’re playing in cqb where the maximum distance is maybe 10m, well there’s not much point spending money getting a setup that can shoot well beyond that if you’re not going to be using it, you’d be much better spending money on grenades.
So what weight should you run? Well as heavy as you can afford and your gun can lift. Personally I use .3’s for all my “assault” type guns, .2’s in my mg42 (even though it shoots lovely on .3’s) and .43’s in my vsr, although should I ever sell my soul and play indoors I’ll just run .2’s.
Part 5- energy
So our final ingredient is energy, most folks will know this as fps because when we’re getting our guns checked at the local site that’s what we get told “oh sorry you’re 400fps and the limit is 350 so you can’t play”.
Well what’s really important is energy, kinetic energy, which is a factor of our bb mass and it’s velocity, it’s why the limits are different for different weights of bb’s and why it’s a common tactic of the cheater to rock up at the chrono and say “yeah I’m running .2’s” and to chrono at 328fps while anyone with a lick of sense can hear the smack of the bb’s against the backstop and think “like hell is he running .2’s”
This is where I take exception to the argument of “marui’s shoot fine at 328fps, you don’t need fps to get range”, well you do, or at least you should try to get your gun shooting as close to your site limit as you can.
The short argument is energy is energy and more is always better, the long version is that energy gives us the velocity that means we need less backspin and can run heavier bb’s without them being really slow and letting our targets stroll out of the path of our righteous pewing.
Remember once the bb leaves the barrel it only has 3 things, and mass is the only one it’ll not lose as it flies downrange. As your bb slows down in the wind it loses energy so the more of it you have at the start the better, it’s why all the cheaters want to run hot.
A friend of mine recently lent me his m16 to do a bit of work on, he was keeping up in terms of range with most of the folk he was shooting at in part due to a good hop, but he’d rage daily at people not taking hits until we chrono’d it to find it shooting at half the site energy, turned out his floating bb’s were hitting with so little force if he was at any distance people just weren’t feeling it.
Now to digress from external ballistics for a second (well, more like 10 minutes) to talk about joule creep.
Think about it like this, our bb in the barrel gets pushed by gas, and how long it gets pushed for depends on the length of the barrel and how quickly it’s getting pushed.
Joule creep is what happens when you have an “over gassed” gun firing .2’s, whilst you can do this with an aeg or a spring sniper it’s much more common in gas/hpa guns.
Think about it like this, your gun can only push so hard, and a light bb will get up to speed very quickly, once it leaves the barrel then it’s not going to be pushed any more. So imagine we had a longer barrel, well if we push just as hard over a longer distance then the bb will spend more time accelerating and hence be going faster.
Well the other way to make a bb spend more time in the barrel is to make it accelerate slower, now we can do this by making the bb heavier, so for the same force it’ll accelerate less (remember newton’s f=ma? Who says math class in school was useless). Now it’s not going to come out any faster than the light bb, but it doesn’t need to, it’s heavier so doesn’t need to go as fast to have the same energy.
A good way to test this is to take a WE or marui gbb pistol, fill it up with .2’s and chrono it, for the sake of argument lets say it fires at 328fps which is 1j
Now try it with .3’s and it fires 300fps, which is 1.2j or equivalent to firing a .2 going at 370fps
This is why it’s important to chrono a gun on the weight you plan on using in a game, because a player can perfectly honestly, indeed without even realising himself, chrono his gun on .2’s and pass, but then fill his mags with .3’s and spend the rest of the day shooting a hot gun.
Now you can argue all day that “yeah but he passed the chrono”, well it’s the exact same thing as running hot, the effect is the same (ie it hurts more and he has a more range), if everyone on the site does it or is happy with people doing it then the site's limits should be changed to reflect this.
so digression aside what energy should you run? Well my opinion is you should take your site limit in joules and get your gun to shoot that on the ammo you use, any less will be a disadvantage and any more will be cheating.
Part 6- nerd time!
Well, that was a long rant without nerdy equations and whatnot so time to put our science hats on.
So our bb in flight has 3 forces acting on it:
Lift- from our magnus force, this is a product of our backspin and our velocity. The full equation I’ve used is for a cylinder but it’s close enough for showing the effect:
Looks an awful lot like it’s not just backspin and velocity there so lets break it down:
Fl is our lifting force
v is our velocity
ω is our angular velocity (aka how much spin we have)
r is our radius
l is our length (for the cylinder example I stole this equation from)
ρ is the density of air
so if you think about it, if we make some broad assumptions we can say that the spin doesn’t slow down (at least not enough to make a difference) and the density of air doesn’t change enough to make a difference either. Now we’re dealing with the same size of bb so really the only things that can change are F as a product of v and ω.
Drag- this is just from velocity and bb diameter (so sorry marushin guys but not sorry, we’re dealing with 6mm spheres here), although once again we have a more complicated equation:
Ok so that’s not as bad
Fl once again is our force, only this time it’s for drag
Ρ has snuck in again, it’s the same atmospheric density
v is a bit of a cheat, technically it’s the velocity of the air against the bb but I’m not NASA so the velocity of the bb is close enough
Cd is our drag coefficient, again a big simplification but for our purposes we’re saying 0.47 for a sphere that’s got turbulent air around it (little known pseudo fact marui “magic dust” actually bonds the airflow to the bb causing laminar flow dropping the Cd to 0.1! (*not entirely serious))
A is our frontal area of the bb, good ol’ πr²
So making the same assumptions before, with the addition of assuming turbulent flow, we end up with Fd being variable as a result of v.
Gravity- this is from our bb weight and here comes the scary equation:
Wait, what? That was meant to be complicated! Well once again it is, technically g can depend on where on earth you are but in reality nobody cares because it’s close enough. Remember back to school math g is 9.81m/s²
So with that explained on to the bit where I skip the really dull bit and just tell you to go look at the spreadsheet, basically it takes these 3 equations and with the starting parameters you give it it’ll spit out a graph of how different weight bb’s will fly.
So as a quick note on it’s use:
Muzzle energy- I’ve rated this as fps on a .2gbb just because that’s a unit we’re all familiar with, if you want to use joules it’s to the right of the reset hop button
MED distance- this is the distance of your MED, for snipers interested in just how much hurt your 500fps gun running .5g bb’s inflicts on someone at 20m
Energy at MED- this gives you an MED if you want to know how far a bb goes before it slows down to a given energy, useful for showing how far a bb can go whilst still hitting as hard as an equivalent gun point blank.
Reset hop- this was a right pain making the spreadsheet so unfortunately the best I can do is fix the hop setting for a 20cm rise before it drops again, this will change if the fps is different so remember to press that button otherwise you’ll get some suspiciously good range numbers and some strange graphs.
Maximum range- this I’ve set to the distance I’d call the “effective” range, ie when a bb has risen 20cm from where it left the barrel, then drop to 20cm below where it left the barrel, so if you fire at a man sized target you’ll hit him without needing to fire your gun wildly into the air and assuming he’s kind enough to stand there and get shot.
So a couple of interesting points to note:
Say we have a site, regular rifles have no MED and can be 1.3j or 375fps, snipers have 2.3j or 500fps but they have a 20m MED:
Our rifleman running .2’s can shoot 38m, but if he can get enough spin to lob a .5g bb he’ll shoot 52m
Our sniper running .2’s can shoot 43m, and at 20m the hit is a measly 0.4j or the same as being shot point blank by a 205fps gun, thing is if he packs .5’s in his gun he’ll shoot 60m and more importantly at 20m he hits with 1.27 joules, or the same as being hit by the rifleman at point blank
So with that I’ll leave you to have a little play.
Part 7- but I’m not a nerd! What do I need to do to get my gun to shoot far?
So we’ve covered pretty well the science of how to get a bb to go far, it needs to be heavy, with lots of energy, and enough spin and if it has those 3 things it’ll go as far regardless of what gun it comes from.
So why spend money on a fancy polarstar kit with an rhop like my favourite youtuber uses?
Well back at the start we mentioned you need 2 things to get hits, and the first is you need to hit in the first place.
This is where all our upgrades come in, a fancy barrel and hop unit wont make your bb’s go any further, but what it will mean is they’ll all start the same, we’ve all seen that guy with his 50rps dsg build m4 spray a mag load of bb’s only to get salty and rage because “he’s not calling his hits” when in reality by the time it gets to the target it’s a 10 foot spread.
So the first place I’d recommend looking is your hop unit, there’s a million different designs that promise the earth so I’m just going to cover the standard, flat and radial hop and hopefully explain what’s going on so you can understand what some of the fancier designs out there like maple leaf are doing.
So the standard hop is what most guns come with these days, it’s basically a lump of rubber in your barrel:
Now lets add the bb, the trick here is to realise that a barrel is not exactly 6mm and neither is the bb, nothing is perfect and there’s a tolerance. So when you’re buying expensive barrels and bb’s what you’re really paying for is for them to be exactly the size they say they are and in the case of bb’s to be all exactly the same size.
Now a bb is slightly smaller than the barrel, so we can be sure it won’t get stuck, this is why it’s important to keep your barrel and ammo clean because enough dirt and it’ll be rubbing it’s way down messing up that lovely backspin we want.
Now we want our backspin to be the same every time, not just in terms of how much backspin there is but also in terms of the backspin being the same every time:
The perfect scenario is vertical spin like this:
But of course the bb isn’t going to be nice and pop perfectly in the centre every time:
So our spin is at an angle, and as a result our magnus force will also be at an angle, lifting the bb not just up but sideways, this means it’ll try and curve off to the left or right, and because not all the force it acting upwards it’ll also lift up more or less, this is our friend with his ultra fast gun shooting a 10 foot spread and not hitting anything.
So our alternative is to flat hop, and try to minimise this angle:
Which when we try our wonky bb does this:
So we get less angle, our bb’s drift less and our spread is less, meaning our bb’s are much more likely to hit what we’re aiming at, not only that but a flat hop will have a longer contact patch, or in other worse it spends longer spinning the bb, meaning we can spin heavier bb’s and get all the wonderful benefits heavier ammo allows us.
Flat hopping a gun really isn’t as hard as you’d think it would be and can get some excellent results for very little money, and whilst it wont give you more total range on it’s own, it will give you accuracy and the ability to use heavier ammo increasing the range you can hit things.
But what if we want more? More is good right?
Well my friend in steps the Rhop:
Man it took ages to align that and it still looks wonky.
Now this might sound controversial but in my opinion the rhop isn’t really much different to a flathop, but the devil’s in the details.
Firstly it’s going to actively pull the bb’s into the centre, if you look at some of the fancy hop designs (like maple leaf) you’ll see they’re trying to go for the same idea.
The DIY rhop just takes a regular flat hop and pops a cut up square of rubber in the window of the barrel, when it’s turned on it can end up looking very flat inside the barrel, but remember when it deforms it’ll wrap round the bb:
So we get as close as we can get to perfect vertical spin every time, and we have the same longer contact patch the flat hop gave us so we can still lift heavy ammo.
Now remember that a bb’s lift is not just a product of spin, but also its velocity and herein lies the advantage of the rhop.
Hop up rubbers come in different hardness’s, and like formula 1 fans will tell you a soft rubber is going to be grippy and a hard rubber not so grippy, now these aren’t cars but there’s one solid advantage to a hard rubber and that’s stopping leaks- remember your airsoft gun is chucking a lot of air behind that bb and air likes to escape.
Now one way is to go down the barrel and shove the bb out of the way, but if it can shove its way out past the hop rubber or the nozzle or better yet just not go anywhere and stay in this nice cylinder thank you very much then it will.
This is why common advice is to check for air leaks. Now since we’re talking about hops we’ll simply say that if you want a soft grippy rubber to spin your heavy bb’s then it’s going to be easier for the air to squish the rubber out of the way and sneak out, so we want a hard rubber, this is why hard rubber’s tend to be recommended for snipers as it’ll help prevent air leaks, and if you’ve played with the spreadsheet you’ll see that faster bb’s need less spin to lift, so the grip is less relevant if your gun is shooting fast.
Well an rhop gives you the best of both worlds, because the patch and the main body of the rubber are separate you can have a nice hard air sealing rubber on the outside and a nice soft grippy rubber on the inside, yay!
Downside is these can be a bit of a pain to get right, however it’s worth experimenting because the boost in accuracy and by extension effective range is a worthy payoff.
now i haven't covered bb's barrels gearboxes and springs because frankly it's pretty straightforward:
good bb's are consistent in size, weight, surface polish and roundness, check out Gasman's topic if you want to look at that makes up a good bb.
good barrels are straight, there's no variation along their length, tight versus widebore is it's own topic but in my opinion that's secondary to quality (ie if you care enough to consider proper tight/widebores you should really focus on quality too)
good gearboxes don't leak air, so they have good o rings, good tolerances and good compression, apart from that most gearbox discussions are more about supporting higher rates of fire, reliable feeding etc.
and springs give you more fps, pretty self explanatory, pick the right one for your setup and site limits.
most of what makes an hpa system so good is it gives the same pressure and volume of air for every single shot and consistency of fps=consistency of spin=accuracy=effective range.
Part 8- conclusions
Well the primary conclusion from this is that I should really be getting back to doing proper work, and that it’s almost certain I’ve just annoyed several strangers on the internet with my wonky diagrams and half-baked ponderings who will proceed to tell me I’m wrong about something.
Now I’m not trying to lie to you, and if I am wrong I’d like to know because at least half of what I know has been learned through internet arguments.
hell I used to think running .2’s was the best way to go for low fps until a bunch of strangers on the internet told me I was wrong. When I properly tried it to prove to myself I was right I realised I was wrong, and now look at me preaching the benefits of heavy ammo, with science!
The best way to learn this stuff is to try it, it might be scary to crack open your first hop unit and cut it up with a pair of snips to make a flat hop but if you’re too scared to do it the first time you’ll never be able to try it a second or third time and eventually get it right.
But if you think I’ve made a mistake, or I’m spreading misinformation then please explain yourself so I can understand what the problem is and either admit i'm wrong and learn something, or prove you wrong and you'll learn something, either way someone gets to learn something and:
now if anyone asks, i've been researching de-lamination of fibre matrix composites all morning ;)
BB range study.xlsm