A few articles about terminal performance of bullets from DOC GKR and others relating velocity and barrel length.- edited to pick out a few paragraphs. Velocity matters, unless you're just shooting paper.
https://www.itstactical.com/warcom/ammunition/military-ammunition-failures-and-solutions/
https://www.justice.gov.za/comm-mrk/exhibits/Exhibit-JJJ-112.pdf
http://www.mlefiaa.org/files/ERPR/Terminal_Ballistic_Performance.pdf
https://www.realcleardefense.com/ar...ich_service_had_the_better_bullet_114140.html
"Criticism
There has been much criticism of the poor performance of the bullet on target, especially the
first-shot kill rate when the muzzle velocity of the firearms used and the downrange bullet
deceleration do not achieve the minimally required terminal velocity at the target to cause
fragmentation.[19] This wounding problem has been cited in incidents beginning in the first
Gulf war, Somalia, and in the current conflicts in Iraq and Afghanistan. In recent lab testing
of M855, it has been shown that the bullets do not fragment reliably or consistently from
round-to-round, displaying widely variable performance. In several cases, yawing did not
begin until 7–10 in of penetration. This was with all rounds coming from the same
manufacturer.[19] This lack of wounding capacity typically becomes an increasingly
significant issue as range increases (e.g., ranges over 50 m when using an M4 or 200 m when
using an M16) or when penetrating heavy clothing, but
this problem is compounded in
shorter-barreled weapons. The 14.5 inches (37 cm) barrel of the U.S. military's M4 carbine
generates considerably less initial velocity than the longer 20" barrel found on the M16, and
terminal performance can be a particular problem with the M4.
Combat operations the past few months have again highlighted terminal performance
deficiencies with 5.56×45mm 62 gr. M855 FMJ. These problems have primarily been
manifested as inadequate incapacitation of enemy forces despite them being hit multiple times
by M855 bullets. These failures appear to be associated with the bullets exiting the body of
the enemy soldier without yawing or fragmenting.
This failure to yaw and fragment can be caused by reduced impact velocities as when fired
from short barrel weapons or when the range increases. It can also occur when the bullets pass
through only minimal tissue, such as a limb or the torso of a thin, small statured individual, as
the bullet may exit the body before it has a chance to yaw and fragment. "
"Unfortunately, combat operations since late 2001 have again highlighted terminal performance problems, generally manifested as failures to rapidly incapacitate opponents, during combat engagements when M855 62 gr “Green Tip” FMJ is fired from 5.56 mm rifles and carbines. This is not surprising,
since M855 was not originally intended for use in carbines or rifles, especially those with short barrels."
"This failure of 5.56 mm bullets to upset can be caused by reduced impact velocities when hitting targets at longer ranges, as well as by the decreased muzzle velocity when using short barrel carbines. Failure to upset can also occur when bullets pass through minimal tissue, such as a limb or the torso of a thin, small statured individual, as the bullet may exit the body before it has a chance to upset. Finally, bullet design and construction plays a major role in reliable bullet upset. Without consistent bullet upset, wounding effects are decreased, rapid incapacitation is unlikely, and enemy combatants may continue to pose a threat to friendly forces and innocent civilians."
"
These carbines had shorter barrels, usually around 14.5 or sixteen inches, which gave them lower muzzle velocity compared to the full twenty-inch barrel of the M16. The lower velocity of these carbines led to incidents such as those in Mogadishu where the M855 fired out of a CAR-15 failed to fragment and put targets down reliably.
The issue of M855 lethality continued to plague the military in the post-9/11 era. As the wars in Iraq and Afghanistan dragged on, the U.S. Army and Marines began to issue out M4 Carbines, which use a 14.5-inch barrel, in larger numbers. This only exacerbated the problem with M855. "
Terminal Ballistic Performance of the 5.56mm Cartridge
"
If using a short-barreled weapon: The same guidelines apply as for barrier penetration loads. SBRs
usually have insufficient velocity to achieve fragmentation velocity."
