Cylinder Gap

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.  Consider these points:

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.

Below you will find a list of the 13 data sets for the different ammunition brands/loadings tested.  Each one links to a page showing its performance with the three different cylinder gaps and over barrel lengths from 18" down to 2".  Each ammunition brand/loading tested is listed across the top, with barrel lengths down the side.  You will also find a graph to show the trend for each cylinder gap over the different barrel lengths.

In order to be able to directly compare the performance curves for each ammunition brand/loading to the next, S. W. McPherson has come up with an Excel chart which allows you to compare any combination of cartridges tested in the Cylinder Gap tests in 2011 on a single graph.  You can find the Comparison Chart for the Gap Test Data on the "Raw Data" page.  If you would like more detailed information showing the actual data collected for each test sequence, download the pdf or Excel files, also linked on the "Raw Data" page.

Got a question?  Check the FAQ.

Magtech, .38 special CBC short, 125 gr. LRN

Ultramax, .38 special, 125 gr. RNFP

Fiocchi, .38 special, 125 gr. XTPHP

Cor-Bon, .38 special +P, 125 gr. JHP

Buffalo Bore hvy, .38 special +P, 125 gr. LV Gold Dot JHC

Cor-Bon, .357 magnum, 125 gr. DPX

Speer LE, .38 special +P, 135 gr. GDHP

Black Hills, .38 long Colt, 158 gr. RNL

Black Hills, .357 magnum, 158 gr. CNL

Buffalo Bore, .38 special, 158 gr. soft lead SWC-HC (std. pressure short barrel low flash hvy)

Buffalo Bore hvy, .38 special +P, 158 gr. LSWHC-GC

Federal Premium, .357 magnum, 158 gr. Hydra-Shok JHP

Magtech, .357 magnum, 158 gr. LSWC (357C)

Real world guns tested

Other Resources

BBTI is not the end-all of ballistics testing, just one more component available for the common good.  In addition to extensive discussion about ballistics to be found at many gun forums, here are some other great resources pertaining to ballistics testing you should check out.  (And if you would like to recommend a site to list here, please send an email.)
  • BrassFetcher:  excellent resource, with an emphasis on bullet performance in ballistic gelatin
  • The Box O' Truth:  testing ammo penetration through various barriers
  • Terminal Ballistics Research:  Specializes in the research of cartridge & projectile performance, using hard data gathered from 20 years of hunting game.

Acknowledgements

We'd like to personally and specifically thank Pat Childs at Fin & Feather in Iowa City, as he not only helped get most of our ammunition and other supplies, he was the brilliant gunsmith who worked with us to make this insane project much more practical.  Without his help all of this would have been much more difficult and perhaps impossible.  Anyone who uses our data owes him a debt of gratitude.

And thanks to our spouses, who were not just tolerant but enthusiastically supportive of this rather nutty project.

Disclaimer

This project, and all of its results, is only our fault.  We (well, Jim K, mostly) paid for everything ourselves, and we did not receive any kind of sponsorship or remuneration from anyone.  We did all the work.  We used products we were either familiar with, or because they were what was available, and mentioning them by name does not constitute an endorsement of any kind.  Furthermore, the data is provided purely for entertainment purposes - to better facilitate arguments over what ammo or caliber or gun is "best."  How you use the data is entirely up to you.  And if you think you could do better, feel free to spend the money and do the work and publish your own results.  Or not.  Your choice.