What is it?

Joule creep is something that happens when a heavier BB is found to have a greater level of potential energy (measured in Joules) than a lighter BB in the same RIF.

How does that happen then?

To know that, you need to understand a little of how airsoft RIFs work. All of them are designed to propel a BB through a narrow tube (the inner barrel) using a gas (most of the time air, but for GBB propane) as a means of propulsion.

If the volume of that gas is greater than will fit within the volume of the barrel, then a lighter BB might exit that barrel with some of that volume (or pressure) “unused”. This is known as “over-voluming”.

As a heavier BB travels slower, it has a greater chance to gain energy from this excess volume and so when it leaves the barrel it will have more potential energy.

The end result is that a RIF chrono’d with a .20 gram BB might have a lower energy output than a heavier BB.

Why does it matter?

Well mainly due to ignorance of this phenomenon, there are still many sites that will chrono a RIF on a .20 gram BB, rather than the weight of BB that is intended to be used. It might be under the site limits (or UK law) on a .20 gram BB, but could be over site limits (or laws) with the BB weight that is going to be used.

This is especially dangerous on a DMR or Sniper Rifle build, as its much more likely that the BB weight used will be at least double that of .20 grams. Not only could this potentially break the law, its very dangerous – eye pro and face protection can only stop a projectile with so much energy before it breaks.

If you find a site doing this, don’t be afraid to question them, ignorance is no excuse to risk people’s safety.