I'm a bit confused about the various types of gas systems used in semi-autos.
Please feel free to correct me:
Older guns, like the Winchester .351 SL were "blowback".
"Gas piston" is like the AK, M1A designs.
"Gas impingement" is the type used in the AR/M16 series.
SKS is "piston and tappet".
Rifles like the Remington 742, and the BAR sporting guns are "recoil" operated.

What do I have wrong?
What am I missing?

i'm kinda poor at explaining, but here goes. The blowback means that the recoil generated by the round pushed the bolt back and a spring returns it. gas piston is a system in which gas pressure from the round is bled off through a hole in the barrel into a chamber which holds a rod like piston. the pressure moves the rod which pushes the bolt back and again is spring returned. impingement systems are similar to the piston, except there is no rod. the operating rod is fitted directly into the gas chamber and the pressure blows the operating rod back. the other system i'm familar with is H&K's which is similar to the blowback, but has a roller delay built into it that delays the bolt movement until after the bullet has left the barrel. Hope this helps