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Gaspolster (Ger) Lit. "gas cushion", or shock absorber

I'll have to try this sometime with a brass rod:

"Actually it is not difficult to push the [5.45mm AK-74] bullet through a bore with steel rod and few taps of a light hammer or mallet. "

About "more recent innovations" is among the most clever ones a Russian 5.45 mm bullet for AK-74 assault rifle, by it's "sliding ballast" principle.

Bullet is about miniature of Russian 7.62 mm LPS bullet (= "Lyohkaya Pulya s Serdtse" = "light bullet with core"), a copy of German 7.9 mm S.m.E ball, having a mild steel core inside the boattailed steel jacket, with slight improvements. Ahead of the steel core is a tiny cylinder of lead alloy, containing little percentage of arsenium. Between the steel core and jacket is a thin lead alloy sleeve or "bearing". Tip of the bullet point is empty. (See the drawing). Bullet diameter is considerably smaller than groove diameter of the bore. Actually it is not difficult to push the bullet through a bore with steel rod and few taps of a light hammer or mallet. But when the powder gas strikes on the rear end of steel core, moving it forwards, the bullet's shank expands just like the shank of an old Minié projectile.

Powder gasses leaks into the gap between truncated-cone shaped rear end of steel core and the jacket, accomplishing so-called "Gaspolster effect". The thin lead alloy sleeve between steel core and jacket seals the gap before the Gaspolster (powder gas expansion) proceeds too far towards the bullet point and cause too high chamber pressure by the excessive friction between bullet jacket and the bore. Sleeve itself expands and distends also the jacket, which fills now the rifling grooves. The steel core acts as a valve which prevents leak of lead alloy from the sleeve to the point space of bullet. The lead cylinder ahead of steel core is now squeezed into the conical hollow space of bullet point, leaving just the tip of it still empty.

The X-ray photographs of spent 5.45 mm bullets shows that the lead cylinder (or now a truncated cone) is always more or less asymmetric. Alloy of lead and arsenium (metallic arsenic) squeezed into the conical space get always more or less uneven frontal surface. The bullet is very long when compared to its diameter. A steep twist of rifling is barely able to keep it stabilized in flight, but the muscle tissue of human being or animals (and the transparent tissue simulant; Ballistic Gelatine) is about 800 times as dense as the air. Even the trifling intrinsic asymmetry of bullet is able to cause tumble of it after hit and penetration to depth of just few centimeters.

Muzzle velocity of 5.45 mm bullet is slower than was estimated ca. 20 years ago (about 900 meters per second from AK-74 rifle) and in general the internal ballistics of a cartridge M-74 is like that of .222 Remington rather than 5.56 mm NATO ammo, but the long, slender, pointed and boat-tailed bullet keeps it's velocity very well, owing the high Ballistic Coefficient in its caliber class. Flight of it is said to be "arrow-like", not yawing like flight of pointed flat-based or hollow-based bullets like Russian LP 1908/-10. Even the visible asymmetry of the bullet is unable to deteriorate accuracy, if it is located close to the central axis of bullet and on the point end of it. (The slightest asymmetry on the base end of bullet shall turn it far away from it's trajectory. The fact known more than a hundred years ago). Each bullet with "mutilated" point should, however, have similar air resistance/ Ballistic Coefficient and weight. Intrinsic asymmetry should also be located into the point end of bullet and not too far from the central axis of it, but the Ballistic Coefficient/ air resistance remains unaltered. Weight of the bullets should not be too much variable, of course.
Ed. note: It'll be interesting to see how much more accurate domestic 5.45mm loads will be compared to Russian & E European loads.

Source: http://www.guns.connect.fi/gow/QA18.html