February 23, 2021
My article about the old .38 Special 200-grain “Super Police” loads included a discussion of stability issues. Some readers asked for more on stability, so here you go.
Every bullet will eventually tumble. Where the tumble commences depends on, among other things, what media it encounters, bullet shape, impact velocity, how fast it sheds speed, and how fast it is spinning. A conical bullet only travels nose-on by virtue of artificial spin-stability created by gyroscopic forces. Once artificial stability is lost, the center of mass (CM) leads the charge. If the CM is near or behind the midpoint, the bullet can tumble.
From a terminal ballistics standpoint, tumbling beginning within the first 15cm (about six inches) of tissue may increase the wounding efficiency of a non-expanding bullet. Forty-year-old data from my crime lab testing held the information needed to spot that.
Military research using complex techniques found that a temporary wound cavity volume is proportional to the energy transferred to the first 15cm of gelatin simulant. We developed a streamlined method that gave much faster data gathering at lower cost, while providing additional data for statistical review.
We placed 15cm-gelatin blocks in a dual-chronograph rig, capturing simultaneous entrance and exit velocities for each shot. Then we calculated the corresponding energies, the difference giving relative wound cavity size. The first tests compared our results to the military’s for 9mm Luger and .45 ACP ball ammo, the only handgun cartridges for which they had data from their setup. We nailed it.
Curiosity had me looking at wound tracks in perforated blocks for other clues. I guess that’s what crime lab guys do. The clear gelatin showed clues in the permanent wound cavities. Some tracks of non-expanding bullets showed slight to significant flaring that grew wider toward the exit point. That’s in-target tumbling.
Used gelatin blocks decompose, so I cannot go back and review them. However, 40+ years later, evidence of bullet stability and other factors survives in the data set. Our decision to run proper 10-shot velocity strings instead of rushing the study allows me to still find interesting trends.
We shot more non-expanding bullets from .38 Special than other calibers in establishing our performance baseline. The 158-grain lead roundnose (LRN) .38 Special was ubiquitous in the early 1970s, so we started there, testing almost a dozen varieties to set a temporary cavity baseline.
The stability evidence lurked in what we called the “efficiency,” for lack of a better word. That is the energy transferred to the gelatin divided by the striking energy. A bullet hitting with 150 ft-lbs and transferring 75 ft-lbs to the block has an efficiency of 0.50. Our 10-shot strings showed how much those efficiency values varied, and we could correlate such variation with visually confirmed bullet tumbling in the blocks at the time of testing.
I found that the range of efficiency across 10 test shots—the highest value minus the lowest value—was a useful predictor of tumbling. In confirmed non-tumblers, the range was usually under 0.10; over 0.15, tumbling was highly likely.
At standard velocity, most LRN loads showed a tumble tendency. At the same velocity, most lead SWC bullets stayed nose-on with values under 0.05. Launch the same bullets at +P speeds and most of the LRN became as stable as lead SWCs. The only lead SWCs that showed high numerical ranges with no visual observation of tumbling had sufficient velocity to just initiate expansion.
I uncovered some head-scratchers. Common sense says the two .38 Special 200-grain loads with those long bullets were under-stabilized, yet, posting 0.06 on our “tumble score,” they were very stable in gelatin at 4-inch velocities of 670 fps.
Another was Federal’s original .38 Special 158-grain LRN bullet. It had a deep base cavity like a hollowbase wadcutter. An ogival roundednose added more length. It should be a tumbler. Then, Federal changed to an improved, shorter version with a more conventional bullet base.
At the crime lab, we added both to our testing where the new one behaved like several other LRNs with a tumble score of 0.24. The discontinued version, a very long bullet that should demand a faster twist, scored a low 0.05 both in 2- and 4-inch barrels. Why? The deep hollow base shoved the CM so far forward that the bullet moved like a badminton shuttlecock even while slowing.
Does rifling twist rate play a part? I now wish we’d had more time, guns, and test blocks. However, we had one good test with .45 Colt, shooting the same Remington 250-grain LRN load in a Colt New Service (1:16 twist) and a Smith & Wesson Model 25-5 (1:18.75 twist). Although the longer-barreled New Service had a 30 fps advantage, the Model 25-5 with the slower twist and a bit less speed posted a cavity volume 17 percent larger.
The Model 25-5’s gelatin block showed clear signs of tumbling starting at the 3-inch point where nothing was evident in the New Service’s block. The tumble score for the New Service was 0.12; the Model 25-5’s was 0.17. The wound cavity for the .45 Colt from that S&W Model 25-5 was twice that of the .38 Special LRN and was the largest we recorded for any LRN load.
Being curious types, we also tested some U.S. .38 Special M41 Ball ammo. Its stubby, 130-grain FMJ-RN bullet should be a non-tumbler in tissue from a slow-twist barrel and a low performer. But it wasn’t.
Average striking velocity from our 2.0-inch S&W averaged 766 fps. The tumble indicators were all over the place, with the highest recorded tumble score in all our .38 Special testing (0.31). However, on-target performance was more of a surprise. It produced a cavity 40 percent larger than 158-grain LRN ammo from the same revolver and with good tactical penetration. Sometimes tumbling can have its advantages.
Old data properly gathered never really dies. It still shows patterns useful for future developers. Although it happened at the beginning of modern ammunition evaluation, I remain proud of the work we did in the Dallas Crime Lab and the decisions we made.