October 25, 2024
By Brad Miller, PHD
Things are not always as they seem. That’s the case with barrels. Most of us think the barrel’s groove diameter is cut to match a caliber’s bullets. And vice versa. And they are. Sorta. You see, the prescribed specifications for manufacturers allow for a little slack. And sometimes there are genuine surprises. That’s the case with the 9mm Luger and the .38 Special/.357 Magnum. If you handload, or if you just pay attention to these things, you know that 9mm bullets are 0.355 inch in diameter. That’s the case for jacketed bullets. Plated and lead bullets generally run a wee bit larger at 0.356 inch. If the barrel’s groove diameter is 0.355 inch, jacketed bullets provide a close fit. Plated and lead bullets are slightly oversize, but that’s okay because they are softer and can swage down a little. But here’s the rub. Most 9mm barrels don’t have a 0.355-inch groove diameter. Most are larger. How can that be? Remember I said there is a little slack in the specifications? Let’s take a look at the specifications. (I’m switching to using four decimal points because I will measure everything with a micrometer accurate to 0.0001 inch.)
Barrels Dr. Miller used soft cast bullets to slug the barrels of 9mm pistols and .38/.357 revolvers for this report. SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) sets the dimensional standards for firearms and ammunition in the U.S.A. Their barrel specification for the 9mm Luger is a groove diameter of 0.3550 inch. But in the corner of their diagram of specifications, it reads, “unless otherwise noted all dia [diameters] + 0.0040.” That means the barrel groove diameter can be between 0.3550 inch and 0.3590 inch and still be within proper specifications. The other part of the barrel, what we call the lands (called the bore by SAAMI), is specified as being 0.3460 inch, also with a + 0.0040-inch tolerance. But my focus will be on the groove diameter. Let’s take a look at some barrel measurements. I slugged 17 9mm barrels. Manufacturers were Apex Tactical, Glock, Kart, Para, Lone Wolf, Rock Island Armory, SF3 Solutions, Smith & Wesson, Storm Lake, and Wilson Combat. Barrels were slugged with oversize (at least 0.3580 inch) cast bullets by driving them through the barrel with a brass rod.
Of all the 9mm barrels slugged, only one was smaller than 0.3560 inch. The smallest groove diameter was 0.3553 inch in a Kart barrel. The other 16 barrels had groove diameters greater than 0.3560 inch. Six barrels had a groove diameter over 0.3570 inch. The largest groove diameter was 0.3578 inch in an Apex Tactical barrel. That’s a range of 0.0025 inch from the smallest to the largest, and they were all within SAAMI specifications. That’s impressive consistency considering they are from 10 different manufacturers. Before I go any further, you should know that the SAAMI specifications of 0.3550-inch groove and 0.3460-inch bore apply to other handgun calibers as well. Which ones? Hold on to your hat! 9x23 Winchester, .356 TSW, .380 Auto, .38 Automatic, .38 Super +P, .38 Special (and .38 Special Match), .357 Magnum, and .357 Sig are all the same, and they all have the same 0.0040-inch tolerance (though rifling tolerances are not listed for the 9x23 Winchester).
The obvious eye-openers are the .38 Special and .357 Magnum. We know that their usual bullet diameters are 0.357 inch, so how can they have the same barrel groove diameter as a gun that shoots a 9mm 0.355-inch bullet? Are revolver barrels generally cut to a larger diameter than pistol barrels, even though they have the same specifications? No. I had access to 14 .38/.357-caliber revolvers. They were made by Colt, Rossi, Ruger, and Smith & Wesson. Their groove diameters ranged from 0.3532 inch to 0.3578 inch. So, no, they are not generally cut larger than pistol barrels, based on this limited sample. Now, some of you noticed that my small groove diameter was 0.3532 inch and said, “What the heck, that’s smaller than the 0.3550-inch specification.” Yes, it is. I questioned it too, so I slugged the barrel three times. It’s real. The gun is a Colt Magnum Carry .357 Magnum. I’d read that some Colt barrels are a little undersize, and this one sure is. A second Colt Magnum Carry had a groove diameter of 0.3551 inch, and a Colt SF-VI .38 Special revolver had a groove diameter of 0.3548 inch (it was slugged three times also). The Colts had the three smallest bores of the revolvers I measured.
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The bottom line from slugging 9mm pistol and .38/.357 revolver barrels is that there was no difference in groove diameter other than the slightly undersize Colts. One final detail here: 9mm and .38/.357 barrels have different rates of twist. SAAMI’s 9mm rate of twist is 1:10 (one full rotation in 10 inches), and the .38/.357 twist is 1:18.75. But this is optional, and different manufacturers vary. Okay, just one more thing. Both the 9mm and .38/.357 SAAMI specs indicate a barrel with six grooves. Here again, different manufacturers do their own thing. Many of S&W’s 9mm/.38/.357 barrels that I have are five groove. My Ruger New Model Blackhawk in .357 Magnum has an eight-groove barrel (and a 1:16 twist).
Bullets SAAMI has specifications for bullets, too. Unlike the barrel specifications for these different calibers all being the same, that doesn’t apply to bullets. They’re more variable. Also, the tolerance specification goes in the other direction. Instead of the tolerance being plus (+), the bullet tolerance is minus (-). For example, the 9mm Luger bullet specification is 0.3555 inch - 0.0030 inch. This means a bullet that measures between 0.3525 and 0.3555 is within SAAMI specifications. Also, another variable is the tolerance range. The bullet diameter tolerance is 0.0030 inch for the 9mm Luger, but it might be different for other cartridges. (See the table.) Bullet tolerance is as little as 0.001 inch and as large as 0.006 inch. One specification that might stand out is the bullets for the .38 Special and .357 Magnum. Their SAAMI jacketed bullet spec is 0.3580 inch, not the 0.3570 inch that we’re used to seeing. And the lead bullet is 0.3590 inch. Wadcutter bullets for the .38 Special Match have a spec of 0.3600 inch. These bullets would clearly be oversize in a gun with a barrel groove diameter of 0.3550 inch! Now let’s measure some actual bullets. For this, I used a Hornady 1-inch micrometer calibrated with a Mitutoyo 0.35000-inch gauge block.
Bullets are seldom perfectly round, and even the best bullets can be out of round by a couple of ten-thousandths. Two tables show the measured diameters of 9mm and .38/.357 bullets. Three bullets of each type were measured. Each bullet was measured in at least five different spots, and the measurement shows the smallest and largest diameter for that bullet. For example, the first bullet of the Hornady 9mm 115-grain FMJ measured 0.3549 to 0.3553 inch. This means the smallest measurement was 0.3549 inch, and the largest was 0.3553 inch, so it’s out of round by 0.0004 inch. Recall that SAAMI specs for 9mm bullets are 0.3555 inch - 0.0030 inch. Jacketed 9mm bullets did not exceed 0.3555 inch, with the exception of the Zero bullet, which hovered around 0.3560 inch. The smallest jacketed bullets in my sample were the Noslers, where some bullets did not reach 0.3550 inch even with the largest measurement. Speer’s bullets, which are plated, varied a little, with the 115-grain examples at or below the specification, while the 147-grain bullet was a little larger at up to 0.3560 inch. Berry’s plated bullets were also a little oversize, and they are advertised as being 0.3560 inch.
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I didn’t include cast 9mm bullets here, but they generally are around 0.3560 or larger. Vendors offer 9mm bullets sized anywhere from 0.355 inch to 0.358 inch, depending on the maker. (Note: Bullet diameter measurements will vary with different lot numbers. My measurements apply only to the bullets I had on hand, and they should not be expected to be the same for all other bullets with the same catalog number from different manufacturing runs.) Now for the revolver bullets. Most .38/.357 bullets were very close to the 0.3570 inch we commonly see for this caliber. Speer bullets were a little larger, with their smallest measurement at 0.3574 inch. The smallest of the samples measured was the Winchester bullets, with their smallest measurement in the 0.3562- to 0.3563-inch range. The largest bullets were the Zeros (again), with a small diameter of 0.3578 inch and the large diameter right at 0.3580 inch. Again, I’m not going to list lead .38/.357 bullets, but I will point out that vendors offer a range of sizing options, usually up to 0.358 inch. One exception that I’m aware of is Matt’s Bullets. Their .38/.357 lead bullets are 0.359 inch. I used them for slugging many barrels. Other vendors say they have options for sizing, and you can check with them for your preferred dimensions.
What All This Means What does all this mean? First, the barrel groove diameters of 9mm pistols and .38/.357 revolvers are the same. Bullets for these calibers are different, with .38/.357 bullets 0.002 inch larger than 9mm bullets. Given these numbers, it means that in some guns, bullets might be undersize or oversize for the barrel. Nine millimeter bullet fit depends on what the actual barrel groove diameter measures. Most bullets will be undersize in most barrels, at least that’s the case for the barrels I measured. But this also will be the case for other cartridges in these calibers that specified a 0.3550-inch groove diameter that use 9mm bullets, like the .357 Sig. For example, my two .357 Sig barrels measured 0.3575 inch (Lone Wolf) and 0.3554 inch (KKM Precision). In the case of .38/.357-caliber revolvers, their larger bullet diameters mean they can fit the barrel’s diameter closer. And because their barrel spec is the same as 9mms, revolver bullets can be oversize for some revolver barrels. How important is it to match the bullet size to the barrel’s groove diameter? Are undersize bullets less accurate? Based on my tests with jacketed bullets, I don’t see much difference when 0.355-inch and 0.357-inch bullets are fired in a 0.3571-inch barrel. The story might be different for lead bullets, but I haven’t tested that yet. If the bullets are too big, will that make my gun explode? Clearly, it does not. If it did, guns would be exploding all the time because some barrels’ groove diameters are smaller than the bullets we’re shooting through them. There you have it: 9mm Luger and .38 Special/.357 Magnum barrels are the same, but the bullets aren’t.