From 'Assault Rifle: the Development of the Modern Military Rifle and its Ammunition' by Max Popenker and myself (details on my website):
"When a bullet passes through a human body, it creates permanent and temporary wound channels. The permanent one is slightly wider than the bullet and is the source of most of the injury, the temporary one is wider still but usually closes quickly without causing much damage. The first generation of jacketed military rifle bullets at the end of the 19th Century had rounded noses and parallel sides and were quite stable, following a straight path through the body. This created a very narrow wound channel, with a strong probability of the victim making a quick recovery provided that no vital organs were seriously damaged, and this earned such bullets a reputation for ineffectiveness.
However, as we have seen, modern pointed bullets are inherently unstable because their centre of gravity is much closer to the base than the tip. They will therefore tumble end-to-end on entering a body, before settling down to travelling base-first. This tumbling creates a far wider permanent wound channel (widest where the bullet is travelling sideways in mid-tumble) and is responsible for most of the injury caused. In contrast, commercial hunting bullets are designed to expand on impact, which greatly increases the size of the wound channel, but these are illegal for military use; tumbling achieves a similar effect in a different way. The rate at which a bullet tumbles depends on a number of factors, mainly concerned with the size, shape and composition of the bullet. The British .303 inch Mark VII ball round, used in rifles and MGs in both World Wars, had a light-alloy tip filler, thereby producing a stronger rearward weight bias which caused more rapid tumbling. This was the subject of criticism from Germany, who argued that it was against the spirit of the international Hague convention of 1907 which banned bullets calculated to cause unnecessary suffering.
Other things being equal, small-calibre bullets tend to tumble faster than larger ones, which partly accounts for the reputation for effectiveness achieved by the 5.56 x 45 NATO round. Both M193 and M855 bullets usually start tumbling about 10 cm after penetration and take another 15 cm to complete the manoeuvre. The 7.62 x 51 M80 ball tumbles more slowly, starting at around 15 cm and taking a further 25 cm to complete. The rate of tumbling for the 7.62 x 39 varies considerably depending on the type; the Russian steel-cored ball at first just yaws between 25 and 30 cm and does not complete tumbling until about 50 cm after impact, whereas the Yugoslav M67 bullet, which has a lead core with a hollow tip (and therefore a stronger rearward weight bias) tumbles much more quickly, starting after only 10 cm. The 5.45 x 45 ball (which also has a hollow tip) follows a similar pattern to the Russian 7.62 x 39, except that it commences yawing after only penetrating about 5 cm and has finished tumbling after about 40 cm. It must be stressed that these are all average figures when fired into an homogenous ballistic gelatine designed to mimic accurately the response of human flesh. What actually happens when bullets strike the decidedly non-homogenous human body may vary considerably, and there have been combat reports of 5.56 mm bullets passing straight through a body without tumbling.
Incidentally, it is often stated that the 5.45 mm's hollow tip is designed to bend on impact to encourage tumbling, and this has been demonstrated when the bullet is fired into plasticine (and sometimes occurs when it hits a human target). However, this does not happen when fired into ballistic gelatine. The hollow tip is probably there to keep the weight down despite the bullet having a long, slender nose for external ballistic reasons. It also provides a useful rearward weight bias. One experimental bullet type specifically designed to encourage fast tumbling was the Lőffelspitz or spoon tip, invented by Dr Voss when working for CETME, which has an asymmetric tip; this doesn't affect the external ballistics.
If the bullet hits an unprotected body, it is likely to be most effective if it completes tumbling within about 30 cm, as this is similar to the average thickness of a torso. This may appear to favour the small-calibre rounds, which generally tumble within this distance. However, if the bullet hits something else first (e.g. the enemy's arm) then the bullet will start tumbling before hitting the body, and in these circumstances the 7.62 mm bullets are likely to perform better. Furthermore, small-calibre bullets are more easily stopped by obstacles such as ammunition magazines kept in chest pouches, as has been demonstrated in tests. In any case, the basic wound channel created by the bigger bullet will clearly be larger than with the small calibres, other things being equal.
A further degree of injury occurs with bullets which break up under the stress of tumbling, the multiple fragments heading off in different directions and adding significantly to the wounding effect. Most bullets do not break up, the most famous ones which do being the 5.56 x 45 loadings, both M193 and M855. The US M80 7.62 x 51 does not break up, but the German equivalent has a thinner jacket with a cannelure (a knurled ring around the centre) which does break up and probably inflicts the most severe wounds of any modern military rifle bullet. It should be noted that bullets which strike bone may also cause much more serious injuries, as the bone fragments can act in much the same way as bullet fragments.
The importance of fragmentation to the effectiveness of the 5.56 mm bullets has a bearing on some of the criticism aimed at the current short-barrelled US M4 carbine. Fragmentation only occurs at high impact velocity. The barrel of the M4 is only 14.5 inches (368 mm) long rather than the 20 inch (508 mm) barrel of the standard M16A2, which reduces the muzzle velocity to the point where fragmentation only occurs at very short range. In the normal 510 mm (20 inch) barrel the maximum fragmentation distance is around 150-200 metres (the longer distance being for the M193), but in the short carbine barrels it can be as low as 50-100 metres. However, fragmentation is an accidental effect rather than a specific US military requirement, and it appears that different bullet production batches may perform differently, with some failing to fragment; serious criticisms were expressed about the effectiveness of the M855 'green tip' bullets used during the American action in Somalia. There are also reports from Iraq of combatants continuing to fight despite being hit in the body several times by 5.56 mm bullets at very short range.
The controversy over the effectiveness of the M4 appears to have stimulated the development of improved loadings. The heavy (77 grain / 5.0 g) Mk 262 5.56 mm loading was originally designed for accurate long-range target shooting, but has been found to tumble well and fragment at much lower velocities than the current service rounds. It was used by US Special Forces and the USMC in Iraq in 2003, and may well be adopted more widely. More radically, a new 6.8 x 43 cartridge is being considered, of which more later."
Tony Williams: Military gun and ammunition
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Thanks for all the links.
