May 15, 2019
At the Dallas Crime Lab we were constantly testing new ammunition for law enforcement. We occasionally wandered into queries of how specific firearm types affected terminal performance. One of these was a classic revolver versus a semiauto showdown.
Sturm, Ruger & Co. and Smith & Wesson both introduced 9mm Luger revolvers in the 1980s. One of my co-workers bought an S&W Model 547 9mm Luger revolver with a 3.0-inch barrel; we insisted he hand it over for testing, and he acquiesced. We .45 Auto fans offered dire predictions about 9mm revolver performance because of the 3.0-inch barrel, the barrel/cylinder gap, and a rifling pattern that should present more resistance to the bullet than that in most 9mm semiauto pistols. We were certain that these factors would reduce 9mm Luger velocities enough to demote any effective police hollowpoint loading to the “also-ran” column.
The 9mm benchmark was our reference S&W Model 39 semiautomatic pistol. Anticipating small velocity differences, we planned 20-shot strings for statistical confidence and used our climate-controlled indoor range. Both handguns were well “broken in” from previous use.
We set a velocity baseline with the 4.0-inch-barreled Model 39 and then ran 20 rounds through the Model 547. There was a hush as our Oehler chronograph flashed the results. Individual shots from the stubby revolver were running with the semiauto velocities. With appropriate statistical tests applied, the 3.0-inch revolver posted an average velocity 18 fps faster than the 4.0-inch semiauto. This seemed insanity. The short barrel alone should have cost the Model 547 that contest.
So we repeated the test. The results were almost identical.
We pondered if these results were common to revolvers chambered for other semiauto pistol cartridges.
I fetched my custom Colt Government Model with a tight match-grade barrel and my Colt U.S. Model 1917 revolver, both in .45 ACP, to show the 9mm results were an aberration. The Model 1917 had a half-inch barrel advantage over the Government Model but also had typically “loose” World War I military service tolerances, including a barrel/cylinder gap crowding 0.009 inch. I believed the Government Model’s tight chamber and well-lapped, hard-chrome-plated bore would win this race without starting to sweat.
After putting 20 rounds through each of the two .45 ACP handguns, the 65-year-old surplus revolver beat out the match-capable semiauto pistol by an average of 15 fps.
We had to know why.
We considered a lot of factors. The 9mm Luger 115-grain JHP ammo was probably loaded to 32,000 psi, and the commercial .45 ACP 230-grain FMJ ammo to about 20,000 psi, yet the velocity differences between pistol and revolver were about the same. The cartridges were loaded with very different, non-canister propellants, but that didn’t correlate with the observations.
We considered the cylinder throat diameters of the two revolvers. We pulled bullets from each ammo batch, removed the residual “Black Lucas” sealant that waterproofs the case mouth, then checked their fit to the cylinder throats by sliding bullets through with a dowel.
The Model 547’s cylinder throats were a loose interference fit to the bullets. Some hesitated, but I could move them easily enough with the dowel. The Model 1917 showed that all-important military attribute: “Leave room for dirt.” Bullets didn’t need the dowel to get through the cylinder throats—they fell through. Throat fit wasn’t a common factor.
I no longer have access to the two 9mm handguns for measurements, but I still own both Colts. The Model 1917 has cylinder throat diameters averaging about 0.454 inch, and the .45 ACP FMJ ammo we used was loaded with bullets whose average diameter was 0.4508 inch. There should have been massive gas loss around the jacketed bullets in those throats with resulting velocity loss. Didn’t happen.
While pushing bullets though the Model 547 cylinder with the dowel, I remember thinking, “What a long cylinder throat this gun has!” Then it struck me: freebore.
Freebore is usually a rifle feature. It describes a section of low- or zero-resistance bore between the chamber terminus and the rifling’s origin. Freebore allows use of longer bullets or, in some applications, higher velocity by allowing more acceleration before the bullet must engage the rifling.
A rush submission stopped our musings, and we never got back to it. I don’t have a Model 547 today, but that didn’t stop me from revisiting it. Images of Model 547 cylinders clearly show it shared the cylinder length (1.675 inches) used in S&W K-Frame Magnums with recessed rims. I used one of my favorite revolvers—a .357 Magnum S&W Model 13—as a stand-in, applying known 9mm max case lengths to get a throat length for the Model 547.
That S&W cylinder profile would leave a 9mm Luger cylinder throat length of 0.900 inch, or 54 percent of the total cylinder length. That’s over half. By comparison, the .357 Mag. cylinder on the same profile has a throat that is only 22 percent of the total cylinder length.
The Model 1917 Colt chambered for the .45 ACP has a throat length of 0.777 inch, or 49 percent of the cylinder length. Its commercial counterpart, the Colt New Service in .45 Colt, has throat lengths of 0.411 inch, or 25 percent of total cylinder length.
The 9mm projectiles we tested were 0.550 inch long; the .45 ACP FMJs were 0.675 inch long. In the revolvers, bullets were out of the case in unencumbered free-flight before ever engaging the rifling.
It was apparent that these bullets achieved significant velocity before losing gas at the barrel/cylinder gap and hitting the rifling. The same bullets fired in a semiauto will encounter the resistance of the rifling before they are fully out of the cartridge case and have to make up velocity farther down the bore. It looked like we had an answer. Long throats plus short cartridges mean some revolvers achieve parity with semiauto velocities through a “freebore effect.”
Were I to reopen this quest for answers today, 35 years later, I would fire additional bullet weights unavailable in 1984, try more gun makes like the Ruger 9mm, and add the 10mm Auto. It could be fun.