12.7×55mm STs-130

The 12.7×55mm STs-130 is a Russian military cartridge designed in 2002 to carry a heavy, subsonic bullet accurately and quietly out of a suppressed rifle — the round the VKS bullpup sniper rifle and the earlier VSS-family concept were built around, rather than a rifle built around an existing round. Its large-diameter, heavy-for-calibre bullets travel below the speed of sound (a published 295–300 m/s across its loadings) specifically so the suppressor does not have to fight a supersonic crack, at the cost of the flat trajectory and long practical range a smaller, faster military rifle round would have.

The same case, at 55 mm, is shared with the very different-looking ammunition of the ShAK-12 assault rifle and the RSh-12 revolver — but the two families are not interchangeable. The ShAK-12/RSh-12 “PS-12” loadings seat a shorter bullet for an overall length of about 73 mm, against roughly 97 mm for the VKS’s own STs-130 loads, so a round meant for one weapon will not feed or chamber correctly in the other despite the shared cartridge case.

This is a thinly documented cartridge outside Russian-language sources. No free dimensional standard exists for it — it never went through C.I.P., SAAMI or NATO standardisation — so every dimension here is recorded as estimated from the Wikipedia infobox, itself uncited on Wikipedia. The one figure independently corroborated is the case type and length: a specialist firearms reference (Modern Firearms, writing about the related ASh-12.7 assault pistol) states directly that the case is rimless and 55 mm long, which is used here rather than left as a gap. No manufacturer or military publication was found stating a barrel length for any load, so no muzzle velocity is recorded for any of the five loadings — only their bullet masses, which are themselves uncited on Wikipedia. No maximum pressure figure was found anywhere.