The Essential Guide to HPA in Airsoft
 

Sometimes it feels like the slightest mention of building or buying an HPA gun can cause more experienced airsofters to explode into heated and often passionate debate over their actual benefits and uses, with various esoteric terms and numbers being hurled around. 

But HPA airsoft guns, and their component parts, don’t have to be confusing or really all that complicated to understand. 

They are fairly straightforward and they still offer some significant advantages and disadvantages that airsofters should be aware of before making any decision. 

If you feel you might be interested in exploring the world of compressed air, or just want to broaden your overall airsoft knowledge a bit more, then read on as we discuss HPA in airsoft. 

What Does HPA Stand For In Airsoft?

In airsoft, HPA stands for High-Pressure Air, a way of powering an airsoft gun using an external tank of pressurized air, rather than an internal gas tank or external cartridge filled with green gas or CO2 (as is common with gas blowback airsoft guns), or a gearbox that compresses air inside the gun, such as in an AEG.

How Does An HPA Airsoft Gun Work?

HPA airsoft guns are pneumatic devices, that is they work through a controlled release of compressed air. 

There are different HPA systems out there, for sure, but the essential concept is pretty similar across them all. 

The way HPA works is that an airsoft gun is connected to a tank or bottle filled with highly pressurized air (3000 PSI or 4500 PSI/206 or 310 BAR). 

These tanks have a built-in output pressure of about 300-800 PSI (thanks to what’s called a bottle regulator), which is still far too much for most HPA systems and airsoft guns, and not something you want launching BBs at friends and strangers. 

HPA tanks are therefore connected to an external regulator, which brings the PSI down to a lower, more manageable level (usually to a maximum of 130 PSI at most) and keeps the flow of air consistent and available.

The regulator connects to a hose, which carries the stepped-down pressurized air down a line, a kind of air hose, that is anywhere from 24” (50cm) to 46” (116cm) or even longer and into the gun and its engine. 

With each pull of the trigger, a blast of pressurized air is released and propels a BB through the hop up unit and out the barrel towards a target. 

When a tank finally runs out of air, users take them to a refill station that has specialized air compressor equipment and experienced staff to do so safely. 

These can be found in some airsoft fields, most paintball stores and even dive shops. 

Required HPA Equipment

HPA Tank

The HPA tank is where it all begins.

Like a paintball air tank or even a basic SCUBA tank, an HPA tank is filled with compressed air, typically held at 3000 and 4500 PSI. 

 

Now those numbers might pop out at you as being pretty high pressure for airsoft (or paintball), and you’d be right.

HPA tanks don’t simply release air at 3-4500 PSI, but rather have a bottle/cylinder regulator attached to them that bring the PSI down to a more manageable level, typically between 300 at the low end (Super Low Pressure/SLP tanks) to 950 PSI, with most outputting at 850 PSI.

Since 850 PSI is still very, very powerful for airsoft, most HPA kits use a second stage regulator, which we discuss below, in order not to blow the engine.

One thing to note is that good quality bottle regulators tend to have what’s called a burst disk installed, which is a safety feature that vents the tank in case it is accidentally overfilled, preventing the tank itself from blowing. 

HPA tanks come in different sizes, measured in cubic inches (CI), from small 13 CI tanks to giant 100+ CI tanks. 

As you can imagine, the larger the HPA tank, the more compressed air it can hold at a certain pressure and the more shots you can get out of it. 

On the flipside, the bigger the tank, the heavier and more bulky your rig is going to be. 

How many shots you get out of a tank can be quite variable.

 It depends a great deal on the PSI you’re running through the regulator, the tank’s pressure (3000 or 4500), the tank’s size and a rig’s overall efficiency.

The bigger the tank, the more pressure it’s held under and the lower the regulator PSI (or FPS) the more shots you’ll get out of it.

That said, you can usually get around 1000 shots or so from a 13/3000 (13 CI @ 3000 PSI) running at 100 PSI and 3-4000 from a decent, 68 CI 4500 PSI tank running at 100 PSI. 

HPA Tank Types

HPA tanks are made of a couple different materials, each with different properties. 

Aluminum

HPA tanks made of aluminum tend to be quite a bit cheaper to buy than carbon fiber tanks, but also tend to be heavier.  

They come pressurized at 3000 PSI, so hold a smaller volume of air pound for pound (50% less air than an equally sized carbon fiber tank), so tend to offer fewer shots before needing a refill. 

Carbon Fiber 

HPA tanks made of carbon fiber are a lot more expensive than similarly-sized aluminum ones, which can be an issue for some players, but do hold a lot more air and are a lot lighter to carry around than aluminum, so are a lot easier to use in a game (fewer refills/less sweat equity). 

For those that can afford them, carbon fiber HPA tanks tend to be the preferred option. 

Regulator

With a tank outputting 850 PSI, and a typical HPA engine only being able to take an operational pressure of about 100 PSI, obviously there is a step missing here. 

That step is a secondary regulator. 

These regulators usually attach to the top of the HPA tank and bring the outputted air pressure down to a far more reasonable 50-180-ish PSI…i.e, something that can be used without causing your HPA engine to self-destruct.

They also act to keep the flow of air constant so that the engine doesn’t suddenly face a lack of air pressure. 

Regulators either come with or can be fitted with a Quick Disconnect (QD) Fitting. These come in different sizes (4mm/6mm) to fit different hose diameters and basically lets you attach and detach lines from your rig without releasing air from the tank. 

HPA Line

HPA lines or hoses run from the regulator to the gun.

These come in an assortment of lengths (24”/50cm to 46”/116cm) typically) and diameters (3mm to 6mm) and generally are rated to accommodate a certain amount of pressure before being damaged. 

In terms of length, it’s important to find a good balance between being long enough to handle and move an HPA gun around comfortably, and being too long that the line snags or gets tangled up. 

Most players should do fine with a 36”/91cm line but it really depends on a person’s height, arm span and personal taste. 

In terms of construction, there are a few different HPA line types out there. 

At their simplest, HPA lines can be a simple, easy to find and cheap cut of rubber tubing that’s rated for high pressure air.
Others, such as the Amped line, have a woven, usually nylon, exterior. These are more expensive but also more durable and resistant to tearing and damage. 

 

HPA Engine

The HPA engine is the heart of an HPA build.
 

The HPA Engine essentially converts the rush of pressurized air into something that will consistently fire a 6mm BB through an inner barrel with a squeeze of a trigger.

HPA engines can be built in many different ways, with many different models and generations of models out there to choose from, but the basics tend to remain the same. 

A valve allows a certain quantity of compressed air in, passing through the system and then to the nozzle. 

The nozzle, as with other airsoft guns, passes the air flow from the cylinder chamber through the hop up and out the inner barrel.

One thing to note when dropping an HPA engine into a pre-existing gun is that nozzles can be fairly specific to certain models of airsoft guns, a nozzle might be designed to fit a Classic Army AUG for example.

Like airsoft AEG gearboxes, HPA Engines can be fitted (and many come with) a fire control unit (FCU). 

Like a Mosfet/ETU setup, these connect to and basically electronically control the HPA engine and can provide a lot of different settings for users to play around, depending on the unit in question obviously, with such as:

• Dwell – poppet dwell, holds the solenoid open and controls the volume of air flow in a barrel. Can be optimized for different barrel lengths. 
• Fire modes- providing an HPA rig with settings for multiple-round burst mode and automatic fire
• Closed Bolt mode – the nozzle is pressed forward and keeps a BB loaded between trigger pulls.
• Open Bolt mode- the nozzle is held back, loading and firing a fresh BB with every trigger pull. 
• Rate of Fire adjustment – adjust how many BBs per second the engine will put out on automatic and burst mode 
• And more

Pros And Cons Of HPA Guns In Airsoft

Pros

Can get very high FPS out of these very easily

HPA rigs are capable of very high PSI. As long as the gun itself can handle it, you can usually get a much higher FPS than other options out there, increasing the range and accuracy of the BBs you fire (all else being equal). 

Tunable FPS makes airsoft guns very versatile 

Part of the attraction of an HPA airsoft gun or rig is the ability to adjust the regulator up or down, adjusting PSI (and, ultimately, FPS) up or down with a very fine degree of control. 

Users can take the same gun to an outdoor event, set it up to fire 450 FPS, and then head to an CQB game with the same gun and quickly tune it down to chrono at 300-350 FPS. 

Adding to the versatility of an HPA rig, they are usually outfitted with fire control units (FCUs) that can allow for even greater control, offering users helpful functions such as fire modes and customizable RPS. 

While it’s true that many higher-end AEGs offer limited FPS tuning via spring adjustment, such as with the KWA Ronin T6s , they can’t really provide the same range and fine degree of control that a regulated tank can offer. 

Reliable and require less maintenance 

The simple fact is that HPA guns have fewer internal parts compared to AEGs, i.e. there is less inside that can go wrong.

Most good quality HPA engines are built to pretty high tolerances and are relatively simple, mechanically speaking, while regulators are generally built to specific standards and will generally last some years if used properly.

Compare this to the intricate, often questionably built, system of gears and related parts inside an AEG gearbox, or even a system of tiny nozzles, valves and pistons of a GBB pistol, and the difference can be night and day.

Maintenance, too, is quite a bit simpler with HPAs. 

A properly put together rig mainly requires users to lube and change o-rings every so often (every 10 000 round so so), as well as to re-hydro their tanks every few years, which means bringing them in to a paintball store or dive shop and having the tanks visually inspected and filled with pressurized water to look for any leaks. 

Can have superb trigger response

Trigger response on HPAs tend to be extremely quick and responsive, partially due to the more simplified internals and constant flow of air.

And with advanced FCUs and aftermarket speed triggers, they can be easily set up with a hair-trigger providing users with almost real steel firearm responsiveness. 

Very quiet

Compared to real steel guns, airsoft guns are pretty quiet in general. 

HPA airsoft guns take things a step further.

 

While AEGs can be somewhat quiet the sound of their pistons, gears, cylinders and whirring motors working in unison can be fairly noticeable when they’re fired. 

Similarly, gas airsoft guns are often favored because of the somewhat loud and noticeable noise and action they produce.

Because of their more constant air pressure and few moving parts, HPA rigs tend to be very quiet, with their noise mainly being limited to a quiet pop of air when fired…something that can actually be silenced even further with a mock suppressor, as demonstrated in the video below.

 

As such, HPA rigs let users really stalk around an airsoft field unnoticed, making them a great option for ambushing opponents or for sniping.

Swappable tanks

If you are someone who collects airsoft guns and have multiple HPA airsoft guns, you can often use the same tank system on different guns, letting you swap between guns as needed while keeping costs down. 

More Consistent FPS

Whether controlled by a circuit board and valve or just by a simple pressure valve, HPA guns have a very consistent FPS.

For safety reasons, compressed air valves are very well built and precise and so the amount of air pressure flowing through tends to remain the same. Ultimately, this means that users will only see about a +/- 1-3 FPS during shooting, which is markedly more consistent than the +/- 20 or so FPS that many AEGs or gas guns can be susceptible to.

Further, running on compressed air means they offer more consistency in cold weather compared to CO2 and even green gas guns, suffering far less of a drop in performance when the temperature dips, although obviously you still have to worry about o-rings and internal buckings freezing and cracking in extreme weather.