Bullets don't yaw in flight, because they are spun by rifling. Long bullets are less stable than short bullets and require a steeper rifling twist, but this is otherwise not an issue when the bullet is in the air.
Yes, they DO yaw (moreless, depending on many factors). I've experimented it enough times when checking my targets after FAMAS rifle shooting : some bullets even impact SIDEWAYS.
The FAMAS rifle has the old standard of rifling grooves that turn once in 12 inches (and have only three grooves). What ammunition were you shooting? The newer NATO standard ammunition requires at least a twist of 1 in 9 inches, preferably 1 in 7.
Using a long aluminium bullet instead of a short lead bullet is entirely to make it less stable (and cut a broader wound when it spins around). Using a long aluminium bullet instead of a long lead bullet is to make it lighter and faster, with more energy (the subsonic 5.7mm rounds carry perhaps one-quarter of the kinetic energy of the supersonic rounds).
Are those aluminium-core bullets really longer than lead-core ones ?
Well, yes, since aluminium has a lower density than lead. If the calibre is constant at 0.224 inches and the mass is constant at 31 grains than changing the density of the filler must change the length of the bullet (and push the centre of mass away from the tip).
Changing the length of the bullet both make the bullet less stable (causing it to yaw more quickly) and increases the rate at which energy is dumped when the bullet yaws.
But indeed, a lighter bullet can go faster, and since E = 1/2mv² (v = speed), impact with more energy than a heavier but slower one. However, light bullets are more affected by wind and are less precise, and still, one aim of the P90 is to be able to defeat body armor at ranges up to 200m...
...and they have a flatter trajectory than 9mm SMGs and large capacity magazines to spray bullets from.
DS, do you have any source to your arguments ? I found that debate interesting and would like to know the truth about it. :)
I have not located an official FN source connecting
a) the use of aluminium with
b) the often-quoted gelatin tests which show remarkable tumbling.
You should find such gelatin tests following the sources at the bottom of the wikipedia article on 5.7mm ammunition. Whether this yawing really occurs in practice is a matter of much debate.
a) and b) above are the two most remarkable aspects of the ammunition and since the laws of physics connect them so strongly I can't honestly think of a single other reason to use aluminium.
...
I'm curious to know why if the UFOAI SMG is 5.7mm, the pistol and machine pistols aren't also 5.7mm? 7.62mm is an odd calibre... it is the old standard Russian pistol calibre, using the same cartridge dimensions as the Mauser pistol from 1896. It's quite an antique compared with the 5.7mm.
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DS