Speargun Band Stretch Factor Explained (+ Band Length Calculator & Chart)

Selecting the right elastics for wooden spearguns is critical for achieving both power and accuracy in your underwater pursuits. At Antex, we believe that every component of a speargun influences performance, and elastics are no exception. In this guide, we explore how different types of elastics impact your shot and why our carefully selected SigalSub elastics offer unmatched quality.

When spearfishers talk about rubber bands, two terms appear often: progressive bands and reactive bands.

They sound complicated, but the idea is actually simple.

A reactive band delivers most of its power very quickly.
The acceleration is strong right at the start of the shot.

A progressive band builds power more gradually.
Instead of a sudden burst, the force increases smoothly during the shot.

Both systems can work very well, but they behave differently in the water.

Band manufacturers usually recommend a stretch factor – for example 320%, 350% or similar.

This number tells you how far the band should be stretched compared to its relaxed length.

Many spearfishers assume that stretching bands beyond the recommended factor will give them more power and more range.

In reality, that is usually not true.

Every rubber band has a physical limit to the speed it can transfer to the spear.

You can think of it as a maximum practical speed the system can reach with a normal spear setup (typically 6.5–7.5 mm shafts).

Once this speed is reached, stretching the band even further does not significantly increase the final spear speed.

Instead, other things start happening.

When bands are stretched beyond the recommended factor:

• Recoil increases
• Loading becomes much harder
• Shaft whip becomes more likely
• Accuracy usually gets worse

The spear may accelerate slightly faster at the start, but the final speed is often almost the same.

So instead of gaining useful performance, you mainly gain stress on the system and on yourself.

Manufacturers do not choose stretch factors randomly.

The recommended value is usually the point where the band:

• works efficiently
• delivers stable acceleration
• maintains good accuracy
• remains reasonably easy to load

Going far beyond that point rarely improves real hunting performance.

Speargun elastics may look simple, but they play a crucial role in overall performance.

Understanding the difference between reactive and progressive elastics allows divers to build setups that are both powerful and accurate.

At Antex Dive, we carefully test and select elastics that provide reliable performance in real spearfishing conditions.

For classic spearguns we typically use Sigalsub Reactive Evo, which behaves like a progressive elastic and provides smooth, controlled shots.

For invert roller systems we use Sigalsub Extreme, a reactive elastic that delivers maximum energy and takes advantage of the roller system’s natural recoil reduction.

Combined with the correct stretch factor and proper rubber length, these elastics allow us to create balanced spearguns that perform exactly as intended.

Understanding these principles will help any spearfisher improve their setup and achieve more consistent results underwater.

Determining the correct rubber length is essential for achieving the desired stretch factor.

For traditional spearguns the calculation is relatively simple.

First measure the distance:

from the anchor hole where the rubber is mounted
to the sharkfin tab where the wishbone attaches

Then multiply this distance by 2.

Finally divide the result by the desired stretch factor/100.

This gives the total rubber length before tying the wishbone.

A well-balanced setup is usually better than a “maximum power” setup.

If a band is stretched too much, you may only achieve:

• harder loading
• more recoil
• worse accuracy

—not more range.

Understanding the difference between progressive and reactive bands helps you choose the right behavior for your speargun, while respecting the recommended stretch factor ensures the system performs as intended.

And sometimes, smooth power beats raw power.

• Rinse after every dive: Saltwater can degrade latex over time.
• Store in a cool, dry, dark place: Heat and UV exposure reduce elasticity.
• Check for cracks or wear: Replace elastics showing signs of fatigue to maintain safety and performance.

If you want help choosing the correct rubber length for your speargun, contact Antex Dive.