It's hard to believe that the M855 is inherently more statically stable than the M193 given that the M193 is a simple lead jacketed projectile and the SS109 has the more complex construction of the steel penetrator which would increase the probability of having variations in the center of gravity and would lead to gyroscopic instability and lateral throw off and increased shot group dispersion, etc. This is why Paul Harrell sees better accuracy with the M193 round and he explains that at around
7:30 in his comparison of M193 and M855 youtube video. He is comparing these two rounds at 100 yards however. At 100 yards, the lighter M193 has the advantage and shoots better than the M855. At longer ranges, the heavier SS109 projectile with the better BC has the advantage over the lighter M193 projectile which should translate into better long range accuracy potential except we're talking about military ball ammo which is far from match grade so.
The 1:7 twist for the M855 is pretty tight too which would exacerbate any lateral throwoff issues. Given that the M16A2 was given a 1:7 twist to stabilize the longer M856 tracer round in arctic conditions, not the M855 ball round, the shorter ss109 green tip is actually over stabilized at 1:7 which, as noted, would lead to increased lateral throwoff and increased shotgroup dispersion. The actual required twist rate to stabilize the M855 SS109 is 1:10" so 1:7 seems like quite a bit more than what is required.
The M193 probably has the overall advantage IMO as it
more reliably fragments at typical combat distances. The SS109 has the advantage in penetration of course but even that is not guaranteed as evidenced by observational experiences in Iraq where it failed to penetrate automobile windshields (
but of course, there are plenty of reports of it going right through windshields just fine) and penetration is great for handgun bullets but for .223 rifle bullets, fragmentation is much more preferable.
But some, perhaps many, of us have thousands of these SS109 projectiles lying around so it's probably best to understand where and how they perform well and where and how they perform poorly. And, the bottom line is, for the most part, they kill enemy soldiers pretty darned good and the reports of their failure are exceptions, not necessarily the rule so, I maintain, shooting them into the dirtpile because they aren't M855A1 or M193 and someone on the internet said they weren't any good might not be the best idea. Maybe some of them but I'm not wasting mine.