They tried to "improve" on the original by going to a heavier bullet and speeding up the twist. At 2500 fps a 62 gr from a 1/7 barrel is spinning about 250,000 rpm (no, I didn't break out the calculator. Correction in 5...4...3...2...) Joe.
The "improvement" had much to do with a change in priorities.
M193 was effective within it's 200 meter (it) effective range, but it was not very effective if there were ay intervening barriers, such as glass, walls, etc. In fact, your .45 ACP and 9mm pistol rounds are more likely to pass though an interior walls and an exterior wall in the average house than M193. M193 will start to tumble and fragment as it passes through the first interior drywall and will a lot less likely to penetrate the exterior wall due to the fragmentation. In contrast a 9mm or 45 ACP hollow point will just plug with dry wall and exit the house like a 9mm or 45 ACP FMJ.
M855 was developed using the NATO SS109 projectile with a steel penetrator that was designed to effectively defeat intermediate barriers such as glass, mild steel plate, etc. Specifically, the SS019 round was designed for the FN FNC rifle and the FN Minimi machine guns, and was designed to penetrate 3.5 mm of steel at 600 meters.
That became relevant when the US adopted the M249, an adaptation of the FM Minimi, and the same requirement was imposed on the M16A2 a couple years prior to that.
1-7" rifling was adopted however to accommodate the significantly longer M856 tracer round. The geniuses in charge decided that troops needed to be able to fire the M856 out of the M16A2 and get the same effective range and penetration range. So, while a 1-9 twist was ideal for the 62 gr SS109/M855 projectile, 1-7 was adopted to accommodate a tracer round that seldom gets fired in an M16 or M4, and if it is, it's done over a far shorter effective range.
The effectiveness of M855 on personnel was NOT a primary consideration in the adoption of M855. It is true that M855 will tumble and fragment at the distances described in the table in my prior post, but that's not cut on stone as it's also where the "fleet yaw" issue described below comes into play.
/...One known issue that affects ball rounds is "fleet yaw". Bullets that strike perfectly perpendicular are slow to upset and fragment and may exit before this can take place. Bullets that strike at a significant angle of attack upset and fragment early. This results in inconsistent terminal effects depending on the rifle, ammunition, range, and shot placement.../
The short story is that shorter barrel length and the heavier M855 bullet both limit independently and together the ranges at which you'll get reliable energy dump in the target through tumbling and fragmentation.
But, if you're talking about a self defense shoot at less than 50m, it's pretty much a non issue. And if you're trying to call it a "self defense" shoot at ranges over 50m you better have a really good story and a really good attorney.
