From The New World of Russian Small Arms and Ammo by Charlie Cutshaw;
"9x39mm SP-5 and SP-6
The 9x39mm cartridge was originally developed by TsNIITochmash for use in its AS and VSS suppressed rifles (see Chapter 1). The cartridge is now used in a number of special-purpose Russian weapons, which have been described above, and is even beign considered by at least one U.S. manufacturer as a fesult of its overall effectiveness. There are two versions of the 9x39mm currently in production. The SP-5 is a ball round intended primarily for use in the VSS sniper rifle; the SP-6 armor-piercing round is intended primarily for use in the AS assault rifle. Both use the same princilples to achieve terminal effects.
When a bullet is fired from a firearm, there are normally two elements to the sound that the gun makes; the blast associated with the expanding gases leaving the muzzle and a "crack" as the bullet leaves the muzzle and breaks the sound barrier, all of which ar associated with cartridges that fire bullets at supersonic velocities. A suppressor (erroneously sometimes called a silencer) attenuates only the blast element of the noise. If a bullet leaves the barrel of the weapon at a supersonic velocity, there will still be a miniature "sonic boom" as it breaks the sound barrier leaving the muzzle. For this reason, most suppressed weapons use subsonic ammunition. The nominal velocity of sound at sea level is approximately 1100 fps, depending on temperature, barometric pressure and, other variables, so supsonic cartridges are usually loaded to give velocities apprecialby below this to accommodate for the variables, usually below 1,000 fps. However, when velocity is lost, so are range and energy. In a military cartridge, terminal effects are one of the primary performance criteria, so such a loss is undesirable. Partial accomodation can be realized by loading the cartridge with a heavier bullet, but if range and terminal effects are to be optimized, not only a heavier bullet but also a larger diameter bullet is necessary. This usually raises both ballistic coefficient and sectional density...
Because they have the characteristics mentioned above, these rounds are ideal for use not only in suppressed weapons but in compact assault rifles as well. These weapons are not intended to be used at ranges beyond 400 to 500 meters, and at these distances the SP-5 and SP-6 bullets have very high effectiveness. In fact, I witnessed a SP-6 bullet fired at a standard U.S. Kevlar helmet at 200 meters, and the SP-6 cleanly penetrated both sides. The Russian representatives claim that the bullet will penetrate both sides of such a helmet at 400 meters, and there is little reason to doubt their claim, given the ballistics and makeup of the rounds.
The 250 grain SP-5 ball round is of standard boat-tail configuration witha hollow base and steel-lead core. Ther rear half of the core is lead, and the forward half is steel with a small air space forward of the steel penetrator. The jacket is gilding metal with a laquer seal at the case mouth and primer annulus. The case is laquered steel for both rounds. The cse itself appears to be no more than a 7.62x39 case "necked-up" to accept the relatively large 9mm bullet. 9x39mm cases that the author has informally measured have essentially the same head diameter and case length as their smaller cousins.
The 246 grain SP-6 is quite different from the ball round. The jacket is thin, and the core is entirely taken up by the mild-steel penetrator, except for a lead band surrounding the middle of the penetrator. It has been suggested to me that the band is present to allow for "crush" when the bullet is fired and engages the rifling of the rifle, because bore erosion with early armor-piercing ammunition of similar design was problematic. Whether this is the case reamins to be seen, until such time as discussions can be held with TsNIITochmash regarding their bullet designs."
This is an elcellent reference book. It has information on many obscure Russian designs on both weapons and ammunition. Charlie Cutshaw is supposed to be working on a companion volume also. I would recommend this book to any serious student of small arms.
