AEG volume matching

Automatic electric gun (AEG) volume matching has many benefits for your RIF and really isn’t that complicated to get your head around, if you’re competent enough to disassemble your gearbox, have a chrono and a ruler then you’re good to go!

Some of the benefits are…

  • More energy transfer to your BB
  • Increased range and shot consistency
  • Increased velocity / feet per second (FPS)

Here’s a quick rundown on matching the volume of your gearbox’s cylinder to your inner barrel using a TM 416D Recoil as an example…


For calculating the cylinder volume you need to have access to the cylinder and the cylinder head.


Measure the distance from the back of the cylinder head to the front of the port, on this RIF it is 37.5mm.

The cylinder head of most automatic AEG’s sits inside the cylinder by about 6mm – you’ll have to take note of where your cylinder head sits in relation to the cylinder while it is installed in one of the gearbox halves.


Next measure the inside diameter of the cylinder – on this RIF and pretty much all AEGs it is 24mm.

Using a handy online calculator you can work out the volume of your ‘swept area’, this is the area the piston displaces whilst travelling towards the cylinder head, specifically the area from when the piston head closes of the port until it contacts the cylinder head.

In our case it is 37.5 x 24mm.

Divide the diameter by 2 to  give us the radius; 12mm.

Punch these figures into your cylinder volume calculator and we get a volume of 16965.

Now we need to do the same with our inner barrel – on this 416D the barrel is 275mm long with an inside diameter of 6.08, this gives us a volume of 7984.

Now we simply divide cylinder volume by barrel volume; 16965/7984 = 2.12

What is 2.12?

2.12 is our cylinder to barrel ratio, the piston can push out 2.12 times the volume of the barrel.

How does this help?

If you want to fire heavyweight BBs – which give you the benefit of a flatter flight path and are less affected by crosswind for example – you need more volume / air to accelerate that BB down the barrel.

By testing with your chrono you can see the amount of energy your current setup is capable of and tune it accordingly.


Our 416D fires a 0.2g BB at roughly 300 FPS or 0.84 Joules…


If we fire 0.28g BBs we get a result of 245 FPS or 0.55 Joules, it’s clear to see we have lost energy by firing heavier BBs.

This means not only would the heavier BB likely not travel as far as is possible, it would also be travelling a lot slower, no good for snap shots!

If we increased the barrel ratio we could impart more energy into our BB.

How do we increase the barrel ratio?

We can increase the barrel ratio by using a cylinder with a different port location, one that gives us a greater swept area, or reducing the barrel length but that’s a bit counter productive for this short barrelled RIF!

What is the ideal barrel ratio?

That is the ratio that gives you the highest amount of energy into your chosen BB weight, for instance if you play a lot of larger game sites, your site limit is 328 FPS / 1 Joule (on 0.2g BBs!) you might want to use 0.25g BBs and so you would tune your barrel ratio to give the best energy at that weight.

If you can achieve the same Joule energy with your heavyweight BB as your 0.2g BB you’re doing well.

ROUNDUP: Through testing and finding the ideal barrel ratio you can achieve impressive shot speed and consistency gains for little expense.

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