This is a good question, but easily rectified. In a locked breach semi-auto pistol, the slide/barrel are as one until the barrel tilts/drops/rotates to unlock from the slide. Think of this; The bullet is pulling the barrel forward during firing while the recoil is pushing the slide back. These two forces are equal until the bullet leaves the barrel. Then and only then can the barrel/slide begin the sequence of movement that will allow unlocking.
This is why such pistols are not very ammo sensitive. They work with a wide variety of bullet weights and velocities. They do not have to be "tuned" to one specific load.
Of course an unlocked breach pistol like a High Point or most guns in .25, .32, .380 use simple mass of slide and recoil spring tension to retard the slide movement for a time so pressures "should" be lower upon opening.
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