Fascinating Phenomenon in Cylinder Gap Data

Steve Johnson
by Steve Johnson

During the past few days I have been absorbing the data collected during the BBTI Project’s cylinder gap tests. I noticed a strange anomaly. For each of the cylinder gaps tested, as the barrel length increases so does effect on velocity increases (as compared to having no cylinder gap at all), up until a certain point, then the effect on velocity starts to decrease.

I would have expected that as the barrel length increases, so the gap would have an increasing effect on velocity (because the bullet stays in the barrel longer so the gap will bleed off more gas).

The only explanation I can come up with is because they were using handgun, not rifle, cartridges (.38 Special & .357 Magnum), once the pressure inside the barrel decreased to a certain level the friction from the barrel begins to have a much greater impact on the velocity than the bleeding of gas, thereby decreasing the relative impact of the cylinder gap. Can anyone else come up with any other explanations?

Steve Johnson
Steve Johnson

I founded TFB in 2007 and over 10 years worked tirelessly, with the help of my team, to build it up into the largest gun blog online. I retired as Editor in Chief in 2017. During my decade at TFB I was fortunate to work with the most amazing talented writers and genuinely good people!

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  • Billca Billca on Dec 10, 2011

    I'm not a fluid dynamics engineer nor do I play one on TV. There is a time element here too. As Matt Fulghum pointed out, a short barrel vents the pressure faster than a long barrel and this reduces the effect of the flash gap pressure loss. In a longer barrel, as the bullet propels forward, it leaves behind an expanding gas chamber. This also means the path of least resistance for the gas changes from the flash gap to the barrel as the bullet accelerates through the bore. Initially as the bullet passes the flash gap, the building pressure against the base is vented sideways. But as it moves up the bore, the gas flows straight forward, compressing the gas in the barrel - only excess pressure is vented sideways. This pressure venting at the gap should decrease rapidly as the bullet moves forward.

    Not being a physicist, would not the friction drag become a reducing percentage of the bullet's velocity over time? That is, as the bullet's momentum builds, the friction matters less because it takes less work energy to move the bullet a given length down the barrel. I would think this especially true once the length of the bullet has been grooved by the rifling.

  • TrojanMan TrojanMan on Dec 12, 2011

    It's worth cross-referencing to Ballistics By The Inch here.

    Those guys have done some excellent work. Note that .357Mag has finished accelerating after around 15" to 16" of barrel length, depending upon load. After that, additional barrel length actually slows the bullet down.

    In a revolver, the gap would become inconsequential at that point, hence the trend data we see here.

    In other news, I'm continuously baffled when I see manufacturers putting out lever rifles and carbines chambered in .357Mag with barrels longer than 16". Surely they must know, right?