It's not all plastic and, it is an individual choice, but safe.
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First, the frame is polymer plastic, mixed compounds that make it stronger than just "plastic"..
Second, the frame mounted guide rails are steel, embedded in the polymer.
Third, if one's gun shop didn't like the gun because it was "plastic", then they most likely are from the old school, all metal guns. Some people like and embrace the lighter, more CCW weight-wise, polymer/metal guns.
Fourth, my Px4 40cal has gone 5500 perfect rounds with mostly FMJ 180gr, and around 100 rounds of 180gr JHP, with no failures, just 100%.
And finally, the "half cock" is not a correct term, as it does not go half-way.
It is a "semi-cock" setting that shortens the DA pull by about 20% but, it does make the DA pull shorter,faster, smoother, and easier, along with, it makes thumb-cocking the hammer into SA mode very easy and fast, as opposed to the Hammer fully down in full DA mode.
The real purpose of this semi-cock position is not (found, nor suggested) in the owners manual but, it is referred to as a safety feature, in that, if semi-cocked, or in FULL DA mode, once one begins the DA pull, one will hear a click shortly after the pull starts, as this is to prevent an accidental loss of trigger pull, or thumb-cocking, of the hammer and its sudden drop, hard, back down, but it will stop "at the semi-cocked position", preventing the hammer from striking the firing pin..
Otherwise, a well thought out safety feature that an individual can utilize to decrease a full DA pull or thumb-cock the hammer, much faster and easier, into SA mode, without worry of a human mistake that lets the hammer drop anytime prior to full SA mode click/lock of the hammer.
And I do carry my Px4 in the semi-cocked mode, always.
Ls