The AR15 or M-16 was originally designed with a 1 in 14" twist firing a 55 grain bullet at 3200 or 3250 fps, using stick powder - IMR4198 or a factory equivalent.
That slow twist stabilized the bullet enough in air to keep it accurate to useful battlefield ranges but that marginal stabilization is what caused it to tumble so much when it hit a denser medium, such as flesh.
Most of the rest of your post was spot on but this is wrong. The military did extensive studies and was never able to find any evidence that twist rate had any effect on lethality.
