Commercial ammo in garands test results.

The piston in both systems is exposed basically the same. There is a port and then there is a "gap".

The gap is larger volume wise of course.
There is a huge difference. The area of the gas trap gap is 0.031 774 sq in, the area of the gas port is 0.005 281 sq in, over 6X smaller. And worse, it can get bigger with tolerances and wear.

And there are other differences. The fact that the bullet is still in the barrel during a few mirco-seconds that the gas port is exposed makes a lot of difference.

But, the biggest difference is the gas port not only restricts high pressure gas from entering the cylinder to fast, it prevents high pressure gas from exiting the cylinder to fast. This means that gas expansion, a slower, more controlled form of energy release of around 2,000 psi to atmospheric can occur over about 1000 to 1500 micro seconds. In contrast, the gas trap slaps the piston with a massive pressure of over 4,000 psi for about 30 to 50 micro-seconds, and very little expansion takes place in the cylinder.
 
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