27 January 2020
Absolutely incredible the total amount of misinformation regarding the BCs of 55 gr FMJBT bullets, the external ballistics and their stability out of a 12" twist in the above two posts.
At 3250 fps out of a 12" twist the measured BC of M193 55 gr FMJBTs hovers right at .200. Secondly, at that velocity, the M193 bullet remains sonic until just after 500 yards. A lot of very accurate shooting was done with M16/M16A1s at 500 yards by Army, Marine and civilian shooters. I still shoot a lot of 5.56 loaded with that M193 bullet at 3200 fps out of my 12" twist AR. I regularly shoot it out to 500 yards with excellent accuracy (for milsurp bullets) and when shooting at 600 yards find the accuracy definitely degraded due to the transition to subsonic. I have never had, since '65, had any M193 ammunition "tumble" in flight from any M16/M16A1/AR15 and I have shot thousands of rounds out of them. The 1st time I shot a qualification course (stateside with targets that weren't shooting back) with the M16 was in '66 at Fort Campbell. We shot on the M14 TrainFire qualification course which had targets from 25m to 450m or 500 yards. The course consisted of 120 shots from foxhole, prone, sitting, kneeling, squatting and standing positions. I qualified expert then as I did every year during my career.
Thirdly; the loss of accuracy by a bullet going subsonic became apparent with match shooters shooting 7.62 M118 in M14s and bolt match rifles (most with 12' twists) that had been "Mexican matched" with 168 MKs in the late '60s. Those had muzzle velocities of 2550 - 2580 fps Past 800 yards accuracy went south and 1000 yard scores suffered because they transition to subsonic between 800 and 900 yards. When loaded to 2700+ fps in a 30-06 in an M1 or bolt match rifle the 168 MK will stay sonic to 1000 yards.
Lastly; the idea that all bullets will lose stability when going subsonic is baseless. Only in rare instances or with light weight short bullets for caliber will total stability be lost. Most often it is accuracy that is lost because the bullet is "buffeted" around a bit and wobbles. Because of that buffeting, not a loss of stability, is why accuracy is lost. That is why the bullet, after the transition from sonic to subsonic, will "go back to sleep" and fly on it's merry way point forward still stabilized. Problem is it's direction of flight has been slightly altered so "accuracy" (more correctly; precision) is lost.
How much of a loss of accuracy/precision (or movement away from the original flight path) occurs during the transition to subsonic depends on numerous variables.
Concealment is not cover.........