Muzzle Velocity VS. Barrel Length

Just how much muzzle velocity is lost as we shorten a barrel? Most shooters respond with 50 fps per inch. Is this actually true? When working on an article on the merits of the S&W 4″ .44 Magnum for our sister publication GUNS, I came up with some hard numbers comparing the short-barreled .44 Magnum to the standard 61/2″ S&W. I used Elmer Keith’s standard hard-cast bullet weighing around 250-260 grs. depending upon the alloy, and also his favorite powder for the .44, #2400.

I assembled loads using three powder charges, 20, 21 and 22 grs. of Alliant #2400. With the first charge the muzzle velocities for the short and longer barrel were 1,095 fps and 1,231 fps. With 21 grs. the figures were 1,187 fps and 1,331 fps. Elmer’s favorite load of 22 grs. gave figures of 1,227 fps and 1,380 fps respectively.

So the differences for the 21/2″ difference in barrel length were 136 fps, 144 fps and 153 fps respectively. This is very close to the “expected” 125 fps difference or 50 fps for each inch of barrel. Looking at it another way, the muzzle velocities of the shorter barrel consistently resulted in a decrease of 11 percent in muzzle velocity. The whole point in this study was to see if it was worth packing the extra weight and length afforded by the 61/2″ barrel all day, every day.

How Does It Apply?

Can we expect these figures to be maintained for differing barrel lengths in differing sixguns with differing cartridges? There are many things determining the velocity of a particular load. We have the cartridge itself with a primer, powder charge and bullet. All primers are not equal, all powders are certainly not equal and even if we have the same powder and same charge but from a different lot performance can also be affected.

Then there’s the bullet itself. There are different alloys, different bearing services and different bullet diameters, all of which have their effects. When the cartridge is fired the bullet must overcome the crimp and tension of the cartridge case itself on the bullet, then pass through the cylinder throat, the barrel/cylinder gap, the forcing cone and into the barrel. Different sixguns may have different cylinder throat diameters, sixguns even have different throat diameters in the same cylinder. Sixguns certainly have different barrel/cylinder gap and “identical” sixguns with different barrel lengths may have different sized forcing cones and barrel diameters. Barrels may even have different sized lands and grooves.

I have a pair of Colt Single Action .45’s, one with a 43/4″ barrel and the other 71/2″ and the shorter barrel shot faster than the longer barrel. The culprit turned out to be the oversized barrel/cylinder gap in the longer version. I sent this sixgun off to Hamilton Bowen who set the barrel back with a minimum barrel/cylinder gap and now things are as they should be. Obviously, things can happen to a bullet as it travels from the cylinder through the gap into the barrel. Two Ruger Custom .44 Specials with 71/2″ and 101/2″ barrels should, if they follow the “rule,” differ in MV by 150 fps; yet they differ by only 20 fps. Carrying the extra 3 of barrel only gives a minuscule MV increase.

And The .357

I recently experimented with several sixguns chambered in .357 Magnum. All of these were single actions, with three of them being Colts and the rest Rugers. Barrel lengths consisted of examples of 45/8″, 51/2″, 61/2″ 71/2″, 83/4″, 10″ and 101/2″. Three loads were used, all using Speer bullets. The 158-JHP was loaded with 12.5 grs. of #2400; the 158-TMJ, 12.5 grs. of Accurate #9, and the 146-HP was used with 14.5 grs. of IMR 4227.

The Colt Single Actions consisted of a 2nd Generation 71/2″ New Frontier, a 51/2″ 2nd Generation Single Action Army fitted with a New Frontier barrel and a 1st Generation Single Action Army from 1921. This last had been sent back to Colt some time between 1935 and 1940 to be converted to a 51/2″ .357 Magnum and has been totally customized by King Gun Works (see Sixgunner in this issue).

If our assumption is true, we would lose 50 fps for each inch of barrel loss. This may have been our expectation but there are so many variables when it comes to achieving muzzle velocity this was not to be the case.

With the Speer 158-JHP loaded over 12.5 grs. of #2400, the 71/2″ New Frontier clocked out at 1,201 fps, leading us to expect the two 51/2″ sixguns to be right at 1,100 fps. Not even close! The 51/2″ with the New Frontier barrel clocked out the very same muzzle velocity of 1,201 fps, while the King Colt Custom came in at a relatively slow 1,030 fps — or 170 fps less than the other two.

With the other two loads results were almost the same. With the load mentioned using IMR 4227 the difference in the first two sixguns turned out to be only 7 fps, while the King Colt Custom was much closer to the first two with a difference of less than 20 fps. Two different loads, two different bullets, two different sets of results.

So Where Are We?

The obvious answer is there are too many variables to predict how much we’re going to lose in muzzle velocity for each inch of barrel removed. We don’t even know if the longer barrel will outpace the shorter barrel until we actually shoot them with different loads. In the past I remember some experiments carried on by writers who started with a long barrel, chronographed a load, and then proceeded to remove 1″ at a time recording the results. The only conclusion we can draw from this type of experiment is what actually happens with that particular sixgun. Then, of course, once we have cut the barrel into pieces that particular sixgun no longer exists.

One thing we can say for sure is short barrels don’t always shoot slower than longer barrels and this might be a factor in choosing what barrel length we decide to pack. However, this only covers muzzle velocity and there are other factors to consider when choosing barrel lengths such as sight radius. Longer barrels are usually easier to shoot for most of us. And there’s also the felt recoil factor. One thing I do know for sure is not how many feet per second will be lost or gained by different barrel lengths — unless I actually find out for myself by shooting each individual sixgun. There are no set formulas.

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