“Well, have you tried adjusting your hop up?”
Those are often some of the first words that those new to airsoft hear when they wonder why their brand new airsoft M4 can’t seem to fire consistently past 50 feet.
A critical component in most good quality airsoft guns, a hop up unit imparts the essential spin to a BB that lets it travel farther and more accurately once it leaves the barrel.
A hot topic of discussion in most airsoft boards, and a very common side project for tinkerers and wannabe snipers, the world of airsoft hop ups can be a sometimes confusing place for newcomers.
The many different options, products, specs and modifications out there, often means that hop ups can take on something of a mysterious, sometimes mystical and impenetrable quality in the mind of some airsoft enthusiasts.
Yet in reality they are fairly simple devices that operate on fairly basic principles of physics and are something that we feel most airsofters should know something about, especially considering how many companies are out there promising to help players dominate the field through whatever “ultimate in hop up technology” they’ve come up with.
To help sort through some of the confusion, we’ve created this guide to help explain some of the fundamentals of airsoft hop ups, including their components, how they work, their adjustment and some things that players might want to consider when buying or modifying one.
What is an Airsoft Hop Up and How Do They Work?
Airsoft BBs are light and spherical and aren’t exactly very aerodynamic. Without a little help, they tend to fall to the ground pretty quickly.
An airsoft hop up, or hop, is designed to help airsoft BBs travel further and be a little more effective.
A chambered device located just before the entrance to the inner barrel, airsoft hop ups provide essential backspin to a BB.
This backspin, through the laws of aerodynamics which we will discuss below, provides a slight upward force that lifts the BB, letting it fly flatter and further than it normally would.
As a result, hop up units increase the effective range of the airsoft guns they are attached to.
Magnus Effect
The Magnus effect is how all this really works.
The Magnus effect is a phenomenon most commonly seen in ball sports, where spherical or cylindrical objects pass through the air towards some kind of target.
The long and short of it is that as an object moves through the air, air flows around it like you might see in a wind-tunnel demonstration.
When backspin is applied to the top of a spherical or cylindrical object, like a BB, the air-flow is deflected downwards.
At this point, Newton’s Third Law (i.e. for every action there is an equal and opposite reaction) comes into play.
As the back spinning BB “pushes” the air on its top side downward, the air coming from the bottom effectively “pushes” the BB back up, lifting it and letting it travel further.
In other words, a BB of a given weight that would normally start dropping to the ground at, say, 50 feet will continue staying relatively flat, moving forwards and traveling a greater distance to hit a target at, say, 80 feet instead.
The greater the backspin, the greater the disruption to the airflow on the topside, the greater the lift provided by the opposite reaction.
Physics.
For a more detailed explanation and demonstration, you can check out the video below.
How Hop Ups Work: A Closer Look
To start, a BB enters the hop up chamber, which is located just before the inner barrel’s opening.
The hop assembly includes a small, rubber sheath called a bucking.
The bucking fits over the entrance of the inner barrel, which typically has a little window or opening cut out of it for this purpose.
The inside of the bucking has a little mound molded into it that sticks down into the inner barrel through the window.
Sitting just over the inner barrel window, and on the other side of the rubber bucking, is what’s called a hop up nub.
The hop up nub is attached to an arm assembly, called the hop arm.
This hop arm pushes the nub down, which in turn presses down on the rubber bucking and pushes it partially through the cut out port into the inner barrel itself.
When a BB passes through on its way out of the airsoft gun, its top half hits the bucking’s mound.
The bucking rubber impacting the top side of the BB as it moves forward provides a little friction, disrupting its motion and causing it to quickly spin backwards (backspin).
The now-back spinning BB continues down the inner barrel and out towards its destination.
The harder the arm and nub can press on the bucking, i.e. the more pressure they impart, the more friction and backspin it imparts to the BB, which in turn allows the BB to travel greater distances.
Components of a Hop Up Unit
Hop Chamber / Hop Unit
The Hop Chamber or Hop Unit is the overall L-shaped body of the Hop up unit.
Essentially, these contain and protect all of the functional components and serve to receive the BB and contain the air pressure that will send it through the system.
Most often, stock hop chambers are made of plastic, which is usually ok.
Upgraded units can be made of more sturdy, reinforced polymer or even metal for extra durability, but the important thing is that the Hop Chamber fits the airsoft gun and forms a tight air seal.
The Hop Chamber should fit around the air nozzle tightly, and prevent any air from escaping, which would reduce your FPS and ultimately defeat the purpose of the whole device.
Any hop up adjustments are usually found on the outside of the hop up unit.
Hop Up Arm
The hop up arm is an internal component to the hop up to which the hop up nub is attached.
The arm applies a downward force or pressure to the nub, pressing it into the rubber bucking and causing it to bulge into the inner barrel chamber.
Very simply, pushing the arm down lowers the nub, placing more focused pressure on the bucking, which in turn increases the backspin of BB and the distance it will likely travel.
Raising the arm has the opposite effect, lifting the nub and reducing the pressure on bucking, reducing the bulge in the inner barrel chambler, reducing backspin and the distance the BB is likely to travel.
Usually made of plastic, hop up arms can be made of more durable materials, such as aluminum or reinforced polymer, which can let them withstand more FPS and/or hop pressure without breaking.
Hop Up Nub
The nub is considered one of the more critical components of an airsoft hop up.
The hop up nub is what pushes down on the bucking to help create backspin.
Hop up nubs are usually made out of soft plastic or rubber, so they can apply pressure without wearing through the rubber bucking.
Nubs often come more or less flat but can come in different shapes, which in turn can vary the amount, area and location of pressure that is applied to the bucking, which in turn can affect the BB’s backspin .
Concave and H nubs, for example, are designed (in theory) to apply pressure to either side of the BB for a more consistent and even backspin, reducing the amount of horizontal inaccuracy a BB may have from pressure being applied more to one side than another.
Airsoft Bucking
The bucking is the rubber sheath that is placed on the opening of the inner barrel.
In most builds it is what will provide the BB with backspin by bulging into the inner barrel.
It also is important in creating a proper airseal, as its lips should fit over the nozzle tightly and it should fit snugly in the hop up chamber.
Bucking contact points
Essentially a little rubber tube, the inside of a bucking isn’t usually smooth.
There is usually a small projection or contact point on the inside designed to extend down the inner barrel when pressed down upon by the nub.
These can be any number of shapes and textures dreamed up by manufacturers to add pressure on the BB, all of which, of course, claim to improve their backspin.
There are, generally, two kinds of overall geometries used – a mound of some kind (often a horizontal square) or a pronged/fanged version that tries to grip the passing BB at two points, ideally for more accuracy and consistency.
Bucking Rubber Quality
Buckings are made of rubber and can vary in their thickness and level of softness.
A bucking made of soft rubber tends to seal better around the gun’s air nozzle due to its greater ease of deformation, i.e. squeezing around the surface, and by and large they tend to do well with lower FPS airsoft guns and builds, such as those under 400 FPS.
Because they defore better, softer buckings tend to grip the BB better, providing more backspin relative to the pressure applied.
Finally, softer buckings tend to do better in colder weather, remaining more pliant and grippy where harder buckings become a little more brittle.
Once you start getting into more powerful airsoft builds, however, soft rubber buckings do tend to wear faster and risk tears or blow outs, which will cause a rapid decrease in air pressure.
As a result, higher FPS builds (sniper, DMRs) tend to do better when paired with harder rubber buckings.
Understanding Rubber Hardness Degrees
One thing that comes up frequently in airsoft forums when discussing bucking hardness is their degree.
When airsofters talk about a bucking’s degree, its refers to the relative hardness of rubber on the Shore Hardness Scale
Measured on a durometer, the scale runs from Extra Soft to Extra Hard and is quantified by a 0-100 scale, with 0 being something like a gel and 90 being something like a polyurethane tube.
To make things a little easier to understand, here are some more easily understood shore measurements that you can go out and feel for yourself to see how soft and pliant they are:
| Degree | Example |
| 20 | Rubber band |
| 40 | Pencil eraser tip |
| 60 | Car tire tread |
| 70 | Rubber shoe sole |
| 100 | Shopping cart wheel |
With airsoft, buckings tend to come between 50 and 80 degrees.
Those in the 50-60 range tend to be considered soft.
Those in the 60-70 range are considered medium.
Those over 70 tend to be considered hard.
Flat Hops and R-Hops: Common Hop Up Modifications
Aside from the typical hop up setups, there are a few other ways that airsofters tend to modify their hop ups to get more pressure and more consistency to their backspin.
The most common of these are flat hops and R-hops.
Flat Hops
As we mentioned previously, buckings aren’t generally smooth on the inside and usually have some kind of mound protruding downwards into the inner barrel that provides a contact point (or several) to try and grip the top of the BB as it passes.
With a flat hop setup, the user makes sure that the bucking they are using is smooth (flat) on the inside.
This can be done by either sanding and shaving the bucking’s mound flat or, more commonly, the user simply buys a flat bucking.
The user then attaches a larger, flat faced hop up nub to their arm.
When pressure is applied, rather than a bulging, rounded mound, a nice flat square shape is pushed into the inner barrel.
The overall idea is that the larger, flat contact patch provides the BB with a larger and more consistent surface area from which to get its backspin yielding better range and accuracy.
R-hops
R-hops follow a similar idea as the flat hop, but are a little more of an involved process and take a more permanent approach to things.
In broad strokes, an R-Hop involves taking the inner barrel and fixing or otherwise gluing a patch of rubber onto the barrel window itself, allowing a concave bit of rubber to always protrude into the barrel.
The concave shape allows for far more points of contact (across the concave surface essentially), which in turn provides far more grip on the BB to provide a stronger backspin.
An added benefit is that the patch now separates the backspin component (the patch) from what is providing a good air seal (the bucking).
Rather than the bucking doing both the backspinning and providing a proper air seal, and therefore needing to balance durability and efficiency, users can now select the bucking that gives them the best air seal for their build and create a patch from whatever rubber will give them the best backspin for the weight of BB they intend to use.
So, you might have a very soft and pliant bucking with a stiffer r-hop patch that can deliver strong backspin.
R-Hops are rather notorious due to being somewhat finicky to install.
Because the patch has to sit flush to the particular inner barrel it is being fixed to, basically the process involves trimming a tiny patch of rubber to math the length and height of the window, as well as matching the inner/outer measurements of the inner barrel pretty exactly and keeping the whole thing centered so as not to throw the BB off course.
Similarly, the arm and nub have to themselves be adapted to provide as even and gradual pressure as possible.
BB weight and Hop Ups
Generally speaking, due to their greater mass, the heavier the BB you use (0.30g, 0.40g, etc) the more pressure a hop up unit will need to get them to backspin properly.
Airsofters who find that they are getting good distance with 0.20s and 0.25s can suddenly find that, when switching to a heavier BB, their range starts dropping quickly.
Assuming that the gun is still firing with plenty of FPS at a heavier weight, players will find that they will need to increase their hop up or even change their hop up unit to one that can offer better grip.
Adjustability of Hop Up Units
With airsoft hop ups playing such an important role in letting BBs fly flat and true across longer distances, it perhaps comes as no surprise that some airsoft guns come with hop up units that can be adjusted to better control a BBs trajectory.
When adjusting the hop of an airsoft gun, what you’re doing is essentially increasing or decreasing the pressure that the nub applies to the bucking, in effect increasing or decreasing the backspin applied to a BB.
Dialling your hop up in will keep the BB hitting around where you’re aiming, preventing it from over- or undershooting the target.
Adjusting a hop up
The first step to adjusting a hop up unit is to find out where the adjuster is located.
With airsoft rifles, generally these are located underneath the replica’s dust cover or wherever the bolt would normally draw back.
With better quality airsoft replicas, where there is a functioning bolt catch, adjustment is pretty simple.
Pull the bolt back, engage the lock (which holds the bolt open) and get to work.
Sadly, there are quite a few airsoft guns out there that have purely decorative bolt catches, requiring the user to hold the bolt back while they adjust the hop up, which can end up being an exercise in juggling and balance.
With airsoft pistols, things can get a little more involved.
With many pistols, especially those based on TM 1911s, hop up units are located under the slide, meaning you’ll have to partially disassemble the pistol before making an adjustment.
Not so great if you find yourself needing to adjust on the fly.
Some more modern airsoft pistols, however, have begun making their hop ups easier to access. Typically, these replicas will allow users to access the hop up adjustor through the gun’s ejection port when the slide is drawn.
Much like airsoft rifles, not every airsoft pistol has a functioning slide lock and so users may need to hold the slide back while they work.
Common Airsoft Adjustment Mechanisms
When it comes to adjusting the hop up, there are a few different kinds of mechanisms that airsoft players may find themselves staring at.
Gears
Often found on AR-platforms (M4s, M16s, HK416, etc), SMGs and some pistols, gear adjusters look like little gears attached to the outside of the hop up.
Using a tool or screwdriver, users turn them clockwise to increase hop, counterclockwise to reduce.
Their simplicity, reliability and compact size make them ideal for use when space is small, and they were one of the earliest hop up adjustment designs.
Usually easy to work with, gear mechanisms don’t offer a lot of fine control and can require a bit of fiddling around and testing to really dial them in.
Slider
Found in a lot of AK replicas, sliders mechanisms are sliding adjustment bars that a user can slide forwards or backwards (often by turning a screw), with the space in between each end representing (very visually) all the different increments a user can select.
Sliding them towards the barrel tends to decrease hop, while sliding the bar towards the stock tends to increase it.
Rotary
Found on most modern M4 variants, AUGs, P90s and more, rotary hop ups have a little dial that users can easily turn, usually without the need of any tool.
They tend to be easy to use and offer fine, even adjustment of the nub pressure beyond that of a gear style mechanism.
Some models even have brightly colored numbers printed on the dial, making adjustments easier to keep track of and more intuitive.
Top Dead Center (TDC)
Top Dead Center adjusters are usually found on (or added to as a mod) airsoft sniper rifles, but can be found on a few other airsoft model types.
With a top dead center hop up chamber, the nub is directly attached to the bottom of an adjusting screw, located (as the name would imply) on the top of the airsoft gun.
TDCs offer extremely fine control over hop but aren’t the easiest to use on the fly, usually needing a tool or an Allen key to operate.
What to look out for with adjustable hop up units
While adjustable hop ups can be extremely useful for airsoft enthusiasts, one thing that users should know is that they can end up re-adjusting themselves.
As users move around or fire their mechanisms can work free and change the hop without users knowing about it, reducing the overall consistency of an airsoft gun.
This is a particular concern with cheaper units, but can also happen to higher quality hop ups if lubricant works its way into them.
Similarly, their mechanisms can get jammed with dirt and debris and are prone to breaking if subjected to undue force.
As a result, if you have an airsoft gun with an adjustable hop up, you should make sure to properly clean their mechanisms every so often and periodically check for any excessive play, damage or jams.
Fixed/Non-Adjustable
Some airsoft guns come with fixed hop ups.
This dramatically limits the ability for a user to adjust their backspin, as well as the ability to practically use different weighted BBs than those specified by the manual.
Fixed hop ups are typically found on older and less expensive airsoft guns.
They can also be found on airsoft guns where the ability to adjust hop either isn’t that much of a concern or would be too expensive to implement, such as with tri-shot airsoft shotguns.
The only real saving grace for fixed hop ups is that they are consistent.
Since they can’t be adjusted there isn’t as much of a need to worry about them falling out of adjustment by accident and they will provide more or less the same backspin (given the correct BB weight) until they get damaged or break.
No Hop Up
Finally, some airsoft guns have no hop up at all.
These airsoft guns don’t put any backspin on a BB, and simply fire them out of the barrel, allowing them to follow their natural arc all the way to the ground.
As a result, the effective range of an airsoft gun with no hop up is pretty limited, generally under 50 feet or so.
Typically, most airsofters are advised to stay away from airsoft guns without hop ups, due to their more limited versatility and usability.
Generally, it is only the cheaper replicas that come without any hop up at all.
That said, some airsoft guns are intended for close range, while others allow practicality to take a back seat to action and style.
As a result, even some more premium airsoft guns may come without a hop up unit.
The most well known example of this is with airsoft revolvers, where the focus is more on their revolving action and their experience, rather than their overall usefulness in-game.
Bottom Line
Hop ups are an important component to an airsoft gun, providing essential backspin that can extend the range and accuracy of a 6mm BB.
Knowing how they work, what their components do and, ultimately, how they can be modified can help users make the most out of their airsoft gun and have a better time on the field.
FAQ
Will Martin – Will has been into airsoft and paintball for well over 10 years, and has done it all – from upgrading and fixing gearboxes as a tech to building custom airsoft loadouts for his friends to supporting off those friends as a DM.