The Fundamentals of Undersize Bullet Accuracy

In my previous bullets and bores article in the May 2024 issue of Shooting Times, I described the similarities and differences of 9mm and .38 Special bullets and barrels. The surprising part was that the SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) prescribed dimensions for the barrels of these calibers are the same, but the bullets are not. That is, 9mm and .38 Special barrels share the same groove diameter specification of 0.355 inch, but the bullet specifications are 0.3555 inch for 9mm jacketed bullets and 0.3580 inch for .38 Special jacketed bullets. Certainly, at first glance, those dimensions make no sense because the bullets don’t match the barrels. That’s right, they don’t. In that previous article I showed that the measured barrel dimensions of multiple barrels in the two calibers are the same. That is, groove diameters were within the range of dimensions of 0.355 + 0.004 inch, meaning between 0.355 and 0.359 inch defined by SAAMI, and I found no difference between 9mm and .38 Special/.357 Magnum barrels. This raises the question of how accurately undersize bullets shoot. If you’re using 0.355-inch bullets in a larger groove diameter, does that ruin accuracy? I tested this.

The Test

I used a Smith & Wesson Model 67 revolver in .38 Special with a 4.0-inch barrel for this comparison. It has a barrel groove diameter of 0.3571 inch, so it was good for comparing the accuracy of 0.355-inch and 0.357-inch bullets. In order to make the comparison as equal as possible, I selected two jacketed bullets that were identical in shape and weight but of different diameters. (It’s not easy to find jacketed bullets that fit this description, even when made by the same manufacturer.) Sierra makes a 0.355-inch 125-grain 9mm JHP bullet that is a twin of the company’s 0.357-inch 125-grain .38 Special/.357 Magnum JHP bullet. Their shapes are the same, with a long bearing surface and a short nose, and with a similar wide hollow point, though the 0.357 bullet’s hollow point looks a little wider with the ones I have. The only obvious difference is the 0.357 bullet has a cannelure, which the 0.355 bullet lacks.

Bullets for this comparison were measured with a micrometer that’s accurate to 0.0001 inch. The 0.355 9mm JHP bullets measured from 0.3547 to 0.3550 inch in diameter. The 0.357 .38/.357 JHP bullets measured from 0.3564 to 0.3568 inch in diameter, not quite coming to a full 0.002-inch difference between the two calibers, but instead a difference from 0.0017 to 0.0018 inch. Still, the 0.355 bullets are a full 0.002-plus inch smaller than the barrel’s 0.3571 groove diameter. These two bullets also are nearly the exact same length. My examples of the 0.355 bullet measured 0.547 inch long, and the 0.357 bullet measured 0.542 inch long, a difference of just 0.005 inch. I seated them to the same depth so that there would be no difference in case space. This meant the 0.355 bullet was loaded 0.005 inch longer in overall length. I tested six loads with these bullets, all with different powders. They included Bullseye, Titegroup, Power Pistol, BE-86, 3N37, and HS-6. Cases were specially prepared for loading the 0.355-inch bullets. The .38 Special brass and reloading dies are designed around 0.357-inch bullets. The smaller 0.355 bullets have little if any neck tension when loaded in .38 Special cases. To correct for this, I sized the cases destined for 0.355 bullets with a Lee .38 Special undersize sizing die. It sizes the case down about 0.002 to 0.003 inch more than a regular sizing die. In addition, the necks were expanded and flared with a 9mm flaring die so that they were not expanded too much. This resulted in good neck tension with the 0.355 bullets. The gun was held in a Ransom Rest, and the target distance was 25 yards. A single 24-shot group was fired with each load. The chambers and bore were cleaned between every 24-shot string.

The Results

The results are shown in the accompanying chart. Most 24-shot groups were around two inches wide, which is quite good for any handgun, and demonstrates that this revolver is capable of precision shooting, a requirement for comparing accuracy. The results show that groups with 0.355 bullets were sometimes bigger, and sometimes smaller, than groups with 0.357 bullets, resulting in no overall difference in accuracy between the two sizes. Bullseye powder produced the largest difference in group size between the two bullet sizes, with the 0.355 group a full inch larger than the 0.357 group. Groups with the 0.355 bullets were about 0.20 inch larger than the 0.357 bullet groups with 3N37 and BE-86. The groups with HS-6 were virtually identical between the two bullet sizes, with a difference of just 0.04 inch. But the 0.355 bullet groups were clearly smaller than the 0.357 groups with Titegroup and Power Pistol (0.42 inch and 0.78 inch, respectively.) Looking at the results, three of the powders had smaller 0.357 bullet groups (Bullseye, 3N37, and BE-86), two had smaller 0.355 bullet groups (Titegroup and Power Pistol), and one was the same (HS-6).

The six-powder average group size shows that the 0.355 bullet average was 2.24 inches, and the 0.357 bullet average was 2.20 inches—virtually the same. The undersize 0.355-inch bullets showed no clear evidence that they were less accurate than the 0.357-inch bullets in this comparison. There was only one powder, Bullseye, that had an obvious size difference in favor of the 0.357 bullets. Other powders showed very small differences (less than a quarter-inch) with the 0.357 bullets. And with two powders the 0.355-inch bullet groups were smaller than those of the 0.357-inch bullets. If the 0.355 bullets were obviously less accurate, one would expect them to be less accurate more frequently and with larger differences. This comparison did not find that undersize jacketed bullets were less accurate than bullets that fit the barrel’s groove diameter more fully. The bullet diameter difference was just 0.0017 to 0.0018 inch, which isn’t much in the first place, and didn’t seem to make an obvious difference to the test gun. Another point to be made is that the 0.355 bullets were 0.002 inch smaller than the gun’s groove diameter yet still produced excellent accuracy. Thus, a close fit to the barrel’s groove size was not a prerequisite for precise shooting with jacketed bullets. 

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