When a cartridge is fired in a blowback action, gas pressure builds, pushing both the bullet and the cartridge case. The bullet goes out the barrel, the cartridge case directly forces the bolt back. As soon as movement has developed clearance between it and the chamber, gas pressure flows around it into the action.
When the gas pressure is channeled into the bolt carrier group, the carrier moves backward as the gas pressure develops. As it moves, the cam pin and cam rotate the bolt unlocking it. Within an inch of rearward movement, the gas key and tube disconnect - and gas is already flowing around the cartridge into the action.
Because the cartridge is pushing against the face of the bolt, the upper contains a channel that prevents the bolt from rotating back - which would jam the lugs against the barrel extension. Examination of uppers shows the cam pin is forced against one side of the channel with force, enough to create pitting and wear that removes anodizing. The Army TM requires that the channel be given "generous lube" in order to reduce friction. POF designed the roller cam pin for the same reason, to reduce friction - caused by pressure from the cartridge case pushing the bolt carrier back.
I don't have a problem with hot, dirty, nasty gas doing the work - what is humorous is that so many think other guns don't do the same thing, and that the AR is somehow prone to debilitating buildup of residue. That strictly depends on the powder used - which was incorrectly specified and corrected decades ago. If significant quantities of residue can coat the bolt carrier group and stop the action cycling, how much more the piston and cylinder on that type? Especially when pistons are only 5 to 10% the size of a bolt carrier group?
Well, in the day, Ball powder did cause problems in M1's and M14's. If hot, nasty, dirty gas is directed anywhere, it will affect those components with the same quantities and force. Pistons aren't immune to the same physics.
If anything, it's been proven for 45 years that the hot, dirty, nasty gases do a good job of making the AR work, as long as the operator understands how to maintain the weapon with it's unique characteristics. DI operation doesn't make the bolt red hot - if anything, the Colt video of sustained firing debunks the whole problem of hot, dirty, nasty gas, as the weapon suffers barrel and gas tube failure - not bolt failure from high temperatures.
The AR suffers failures to feed from primarily bad magazines and bad or damaged ammunition far more than being unclean. It's not a problem of hot, dirty, nasty gasses being directed into the bolt carrier, it's a problem of cheap magazines, damaged mil surplus rejects, and poor operator practices. Keep it lubed, it keeps running, regardless of those who insist that it conform to the mythology perpetuated by non users.
