Chamber pressure, muzzle velocity, and bullet weight are only some of the considerations for semi and full-auto weapon functioning. Several other factors enter into the equation including: operating temperature range (artic through desert conditions), recoil impulse, powder burning rate, gas port pressure, firing rate, weapon fouling/cleanliness/lubrication, and so on.
The SKS sturdy firing pin and extractor design minimizes the plague of auto mechanisms: broken firing pins, extractors, and extraction failures. . Gun actions and ammunition are "tuned" for each other during development, testing, and acceptance. Troop trials help make the weapon "soldierproof". Only after much testing and use is the weapon then accepted for service use.
Thankfully, the SKS gas operating system is robust and designed for rough use and minimum maintenance, and can tolerate a large variation in ammunition quality and climatic extremes. The same is true of the Kalashnikov series. This is not necessarily true of other actions. The US Military changed from an M1 173 grain bullet to the M2 150 grain bullet just prior to WWII because of concern that the heavy bullet ammunition battered the M1 Garand, for example. That same M2 ammo remained type standardized for the rest of the M1 service period and remains standard today. Using 165, 180, or 220 grain .30-06 commercial ammunition can cause malfunctioning, or even damage to the M1 Garand. All this because the system was designed for use with 150 grain bullets at a muzzle velocity of about 2800-2900 fps.
HTH
John
