One of the best, easiest and cheapest airsoft accuracy upgrade you can make is to improve your airsoft gun’s compression consistency.

The consistency of your airsoft gun’s FPS is a major factor in how it performs. Small variances and deviations will drastically affect your gun’s accuracy and range, making them inconsistent and unreliable. This is where checking your airsoft AEG’s compression comes in.

What causes these variations?

Your airsoft accuracy and consistency is predominantly affected by your gun’s ability to compress air and use it to push the BB down the barrel. Your muzzle velocity (fps/joules) will not be consistent if this changes. To first understand where these variations come from, we should probably explain (really quickly) how an AEG works. There are 3 parts of how an AEG works:

  1. Your AEG pulls back a piston in a cylinder
  2. After that, it is forced forward by the power spring
  3. The piston moving forward compresses air and is forces a BB down the barrel.

It’s not quite as simple as that, but for this little guide, it’s all you need to know. You need to make sure everything inside the piston and cylinder (and connected parts) is airtight; if it’s not airtight, air escapes and causes an inconsistent amount of air to push the BB. This results in a varying FPS. This is what’s we call the “compression” of your AEG.

How do you ensure a consistent compression and FPS?

A consistent compression relies on how air-tight the parts of your gearbox are and there are 5 key places where air can escape. I’m going to go through each part and tell you how to perfect the air-seal as a quick and easy airsoft accuracy upgrade.

Piston Head to Cylinder Connection

Starting at the rear-most part of the AEG is the Piston. The piston is the part the compresses the air with the help of the spring.

The piston head is what seals against the cylinder and creates pressure. First things first, the piston head should have an O-ring on it. If it doesn’t have an o-ring I strongly recommend that you purchase a piston head with an o-ring.

O-rings are little rubber rings that help it swim.. I mean seal against the cylinder. Some piston heads are really fancy and have 2 o-rings. Some cylinder heads have ports on them, which funnel some compressed air and use it to push the o-ring against the cylinder wall, creating a better seal.

One of the most important things for these piston heads and o-rings, in general, is lubrication. It’s important that you lubricate these o-rings with silicone oil or grease to ensure that they don’t dry out, rip and cause poor compression. If your piston head is still not sealing against the wall, you may need to replace the o-ring with a slightly larger one. 

Cylinder Head to Cylinder Connection

The next place that air can escape is the connection between the cylinder head and the cylinder. The cylinder is a simple metal tube in which the piston head moves up and down. The cylinder head caps off this tube at one end channels the air into a small gap and provides a pad for the piston head to hit into. Once again, this is sealed with o-rings.

As with all of the o-rings in this guide, you must check them for any damage, cuts or tears and keep them well lubricated. If they have dried out and will not seal (assuming they’re in good condition) leave them soaking in silicon oil for a few hours to rejuvenate them. Since this part doesn’t move too much, a worse case quick fix can be to wrap the head in Teflon tape, in order create a better seal.

Cylinder Head to Air Nozzle Connection

A hidden spot that most people forget to check when doing compression is the cylinder head to air nozzle connection. The cylinder head extends by a small tube and the nozzle slides back and forth over the top of it, this facilitates the loading of the BB. Older or lower quality AEGs may simply have a bare metal connection for this part. If this is the case, I suggest you replace it with one that has an integrated O-ring.

The air-nozzle reciprocates back and forth over the cylinder head and for this reason, can often become dry and brittle. Once again, check for cuts and tears in the o-ring and lubricate well to ensure that the O-ring seals.

Air Nozzle to Hop-up Connection

The next quick airsoft accuracy upgrade you can make is where the air nozzle enters the hop up (whilst pushing a bb into the barrel) and seals against the hop-up rubber.

Understandably, this is one of the harder places to seal as you can not use a lubricant (the lubricant will ruin the effectiveness of the hop-up rubber since it works on friction). Ensure that the hop-up is as tight as possible on the barrel. You can fix this with a few wraps of Teflon tape between the barrel and the hop-up rubber.

Then ensure a good seal between the hop-up rubber and the chamber. You can use lubricant here sparingly, to ensure a snug fit without tearing the rubber. *giggle*.

How to test the compression

A simple way to test the compression of the main gearbox is to press the piston into the cylinder whilst blocking the air nozzle. For instance, if no air escapes, despite your hardest pushes, you’ve got a perfect air seal (in the gearbox).

Next comes the harder part, testing the air seal of the hop-up. There’s no simple way of doing this. The best way that I know of is to simply chrono the gun.

Primarily, if you see FPS variations of more than 5fps, there’s a good chance that the hop-up is not sealing 100% and you should re-visit this area. You can skip the first step and go straight to chronographing, but by doing that, you have no clue where the leak might be.

That’s my go-to airsoft accuracy upgrade, once you’ve done that, you can rest assured that your AEG has great compression. Bear in mind that this is something that will deteriorate over time. As o-rings wear down and lubricant degrades, you will have to re-visit your compression job. Have any questions about airsoft cylinders? Take a look at our Cylinder FAQs.