...you!
How the hell;
...can that be with such a slow moving round?
Here's a write up I found. Can't find any ballistic comparison tests though..
SP-5 and SP-6 subsonic assault rifle ammunition
Next line of development, also initiated by Spetsnaz requirements, also involved silenced weapons, but in more powerful form. Since the effective range of silenced pistols is severely limited, scout and Spetsnaz elements of the Soviet army originally employed AK and AKM rifles, fitted with detachable sound suppressors (silencers) and loaded with special versions of 7.62x39 M43 ammo, known as 7.62x39 US (Umenshennaya Skorost – Low velocity). To achieve subsonic velocity along with acceptable ballistics, these cartridges were loaded with heavier bullets, but its performance was still inadequate. So, during the late 1980s, soviet designers developed improved sub-sonic ammunition, suitable for specially designed automatic weapons. These cartridges, known as SP-5 and SP-6, were based on a 7.62x39 M43 case, necked-out to 9mm, and loaded with heavy, streamlined bullets. The SP-5 cartridge was loaded with standard “ball” bullet with lead core, and was intended for accurate sniper work out to 300-400 meters. The SP-6 cartridge featured an armour-piercing bullet with hardened steel core, which could defeat typical military type body armour at the ranges of up to 300-400 meters. Two weapons were initially developed for this ammunition, both based on the same receiver and gas operated action – VSS sniper rifle and AS assault rifle. Both weapons were selective fired, with integral sound suppressors, and used same magazines with 10- or 20-round capacity. Latter on, several more weapons were developed for 9x39 ammunition, such as SR-3 and 9A-91 compact assault rifles, used by elite Internal Affairs Ministry, Police and State Security units. The one problem, associated with 9x39, as well as with most other special purpose cartridges, is that such ammunition is usually quite expensive. An attempt was made during late 1990s to produce much cheaper 9x39 AP loading, designated as PAB-9. This cartridge featured bullets with stamped (instead of machined) steel cores, as well as increased driving surfaces. As a result, accuracy was poor and barrel wear significantly increased, so this ammunition is apparently no longer in use.
cartridge metric designation bullet weight muzzle velocity
7.62 US 7,62x39 12,5 g / 193 grain 290 m/s / 950 fps
СП-5 9x39 16,8 g / 260 grain 280 m/s / 920 fps
СП-6 9x39 16 g / 247 grain 280 m/s / 920 fps
Modern Firearms - Ammunition - Special purpose cartridges of USSR and Russia
