Ballistic By The Inch Updated

Christmas came early for me this year! The Ballistics By The Inch (BBTI) have done extensive testing to find out how much the revolver cylinder gap (the gap between the cylinder/chamber and the barrel of a revolver) affects performance. They shot and carefully measured over 7000 rounds.

The cylinder gap, and gas escaping from it, can be seen clearly in this photo.

From the BBTI website

For many years people have wondered about the effect of the gap between the cylinder and the barrel (forcing cone) on a revolver. How much energy is lost due to this gap? Intuitively, a larger gap would probably mean a greater loss of energy (and hence bullet velocity), but how much functional difference is there? We decided to test it and see.

This is actually a more complex problem than it might seem at first. In addition to variation caused by the actual amount of space between the cylinder and the barrel (forcing cone), there are a number of factors which could have an effect on the loss of energy through the cylinder gap.

So, we decided to test a pretty wide range of different loads available in the caliber/cartridge of our test platform, an Uberti 1873 Cattleman Single Action Revolver (Colt Single Action Army clone) in .38/.357 magnum. In addition, the gun was modified to allow us to change the cylinder gap through the use of different shims on the barrel (where it screws into the frame) to create a cylinder gap of six thousandths of an inch (0.006″), one thousandth of an inch (0.001″), and “flush” (where the barrel was tightened against the face of the cylinder).

Because we expected the effect to be fairly subtle, we decided to do ten test shots of each ammunition brand/loading for each combination of barrel length and cylinder gap, measuring the velocity of the fired bullet over two chronographs set at 15′. This meant we fired thirty rounds of each ammo type at each 1″ of barrel length, altogether almost 7,000 rounds.

The BBTI team (after two years of “encouragement” from me) have also done barrel length tests of the .22 LR and .223 Remington. I will be referencing their site as I continue to preach the benefits of short barreled rifles 😉

Thanks BBTI 🙂

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • fred johnson

    I like the improvements they’ve done at BBTI. I was thinking that the operators of that website were happy with their prior work being complete. I was happily mistaken about that.

    I also like the muzzle energy charts. I don’t recall BBTI having those before.

    • GeoffH

      The muzzle energy charts were there at least a couple of months ago, before the revamp of the website.

  • DaveR

    Nice! I hope they do this with larger calibers too.

  • ThomasD

    I for one am not surprised by the dramatic differences due to cylinder gap. I’ve always felt this to be the case, not because I’ve done extensive chrono work, but because big cylinder gaps always mean dirtier shooting, and dirtier shooting means poor combustion from LOW PRESSURES.

  • 18D

    Most popular SBR’s are extremely limited in use.

  • Lance

    Doesn’t matter the .357 mag and .44 mag are good enough to remain powerful despite the gas release from the gap. The only revolver to correct this small flaw was the Nagant M1895.

  • Ramsey

    It was interesting looking at the 45 and 50 grain .223. The velocity change was almost linear out to 18″, which shows just how inefficient these loads are in short barrels. I wish they would go out to a 26″ or so. I could imagine a rifle length bullpup really clocking these rounds out there. The 55 grain had really started to plateau at 16″, so longer barrels would not help.

    I hope they get muzzle energy charts up for these two calibers soon. They seem more informative to me and the non-linear energy velocity relationship is hard to visualize.

    • 18D

      @Ramsey- Your right about the heavier loads. They’re much more effective out of shorter barrels. The 55gr and heavier loads tend to drop off after barrel length exceeds 20in. As a matter of fact, the 55gr and 62gr loads will actually lose velocity when 20in is exceeded. We used 77gr 5.56 loads in Afghanistan from time to time and they were not only more effective but much easier to make hits at longer ranges. I would like to see charts on heavy loads like the 77gr with longer barrels.

  • Jim March

    This is what I posted over at TheFiringLine on the 38/357 data:

    In this set of tests by Ballistics By The Inch, they rigged an Uberti in .357Mag so that not only could the barrel length be altered, but also the cylinder gap between .006, .001″ and “zero gap”, with shims.

    I do have some issues with their methodology:

    1) They didn’t test .38s in .38Spl chambers – everything was done on a .357. That might matter some.

    2) While their selection of .38 and 38+P ammo was excellent, they have basically ZERO high-performance 357 ammo. The closest they come is Federal’s 158gr load and even that’s pretty wimpy. All the rest are either downloaded (Cor-Bon DPX isn’t full power) or practice grade. There’s not a single high-performance 125gr, even from a major manufacturer. Buffalo Bore was tested in 38 and 38+P, not present in 357. Very annoying.

    3) There’s too many “outlier cases” where in one particular load and barrel length, you’ll get a “flyer” – often in places that don’t make any sense at all. Examples include Cor-Bon’s 38+P 125gr at the 16″ point, Buffalo Bore 38+P 125gr at 3″ where .006″ does well only at 3″, Cor-Bon 357 DPX at 5″ (*** happened there?) and more. The most serious is the Black Hills 38 Long Colt data at 13″ – that spike is just not possible. Damned if I know what happened, but, something did.

    On the plus side, their data for the legendary BB 158+P load starts in the very high 900 range from a 2″:

    Italian barrels aren’t as good for speed as Ruger and later-model S&W wheelgun barrels, so with a better gun than that Uberti you should see BB’s claimed 1,000fps from a 2″ barrel. We also see a huge jump of about 75fps from the 2″ to 3″ barrel in this load – out of a Ruger or recent-vintage S&W we might see 100fps. Oh, and 38Spl chambers might make the extra difference to an honest 2″ barrel 1,000fps load.

    Those issues aside, some clear trends emerge:

    a) The difference between a .001″ gap and .006″ gap isn’t much, but in a 2″ barrel with 38+P ammo the 30fps or so average difference may be enough to cause a round to expand, or not.

    b) The difference between 2″ and 3″ barrel velocities across the various combat rounds is often very high – up around 100fps in some cases. THAT is enough to affect expansion, I guarandamntee it. All of the makers ought to be doing more 3″ guns, and the 3″ Ruger SP101 ought to be selling better than it is. That extra inch of barrel will make a difference in concealment only if you’re doing pocket carry – for anything else (ankle, shoulder, IWB, “belly band”, jacket pocket or fanny pack) a 3″ version will be only fractionally harder to conceal in most cases.

    I have long been a proponent of small gaps in snubby revolvers, and as far as I’m concerned this data supports that concept. I’ve also been pointing out that 3″ ballistics are usually better than 2″ by a useful margin and again, I see a lot of evidence in favor of that idea too.

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