What the cylinder to barrel ratio is
In an electric (AEG) or spring airsoft gun, the piston compresses the air in the cylinder and pushes it through the nozzle into the barrel. The cylinder:barrel ratio compares that available air volume with the inner volume of the barrel the BB has to travel through.
If the air runs out before the BB leaves the barrel, pressure drops behind it and the barrel itself slows it down: this is known as vacuum or suction. If there is a lot of spare air, the BB leaves before the piston finishes its stroke and the excess is wasted, with more noise and wear. That is why a middle range is preferred, usually higher for heavier BBs.
How to measure each value
Use a caliper for the inner diameter of the cylinder and the barrel. For the cylinder length, enter the effective compression stroke (how far the piston head actually travels), not the outer length of the tube. Ported cylinders only compress once the piston head has passed the port, so measure from there.
The nozzle adds its inner volume to the available air. For the barrel, use the full length of the inner barrel, from the hop-up chamber entry to the muzzle.
Reference ranges
| Ratio | Diagnosis | Meaning |
|---|---|---|
| < 1.0 : 1 | Possible vacuum | There is less air volume than barrel volume. The system may run out of air before the end of the stroke. |
| 1.0 – 1.5 : 1 | Low ratio | It may work, but it is below the general reference range. |
| 1.5 – 2.5 : 1 | Balanced ratio | The ratio is within the general reference range. |
| 2.5 – 3.2 : 1 | High ratio | Can be reasonable with heavy BBs, but it is high for general use. |
| > 3.2 : 1 | Over-volume | There is plenty of excess air. Adjust the effective volume or check the result with a chronograph. |
Barrel volume by length
| Barrel length | Barrel volume | Air for 1.5–2.5:1 |
|---|---|---|
| 110 mm | 3,141 mm³ | 4,712 – 7,853 mm³ |
| 141 mm | 4,027 mm³ | 6,040 – 10,067 mm³ |
| 229 mm | 6,540 mm³ | 9,810 – 16,349 mm³ |
| 247 mm | 7,054 mm³ | 10,581 – 17,634 mm³ |
| 275 mm | 7,853 mm³ | 11,780 – 19,633 mm³ |
| 300 mm | 8,567 mm³ | 12,851 – 21,418 mm³ |
| 363 mm | 10,366 mm³ | 15,550 – 25,916 mm³ |
| 407 mm | 11,623 mm³ | 17,435 – 29,058 mm³ |
| 455 mm | 12,994 mm³ | 19,491 – 32,484 mm³ |
| 509 mm | 14,536 mm³ | 21,804 – 36,340 mm³ |
| 590 mm | 16,849 mm³ | 25,274 – 42,123 mm³ |
Calculator limitations
- It is a geometric model: it does not account for leaks, real air compression, friction or the time it takes the BB to clear the hop-up.
- Velocity is estimated assuming the spring delivers constant energy, scaled by the efficiency you enter. Heavy BBs often use the energy a little better, and only the efficiency value reflects that.
- Spring naming (M90, M120…) is not standardised across brands. One brand's M120 may give different results from another's.
- The ranges are general guidelines. Before playing, always measure with a chronograph and follow the FPS and joule limits of your field.