Schengen allows for border crossings with no checkpoints just like US states. Which means the "different states have different laws" canard is no different than Europe. What's to stop someone from leaving Switzerland or Czechia with a firearm until they get caught?
Schengen doesn't have a Texas - Switzerland raised the bar on semi automatics to match Schengen law - it's a more uniform zone than the USofA is wrt gun law.