As I mentioned in my last post - take your upper off of your lower, then pull the charging handle about 2" rearward, then press down on the charging handle. There's no flexion, but you'll see the displacement of the charging handle.

The lugs on the charging handle keep the FRONT end held up into the track in the upper. The rear end, however, doesn't have any lugs, so it's able to float a little up and down.

You can even see this by simply pulling the charging handle fully rearward and locking the bcg open. There's a lot of tailplay in the charging handle. If a guy always pulls back and up on the charger, it won't drag its belly, but if you ever exert any downward pressure - common with AR's wearing a scope - then you'll drag those two track marks across the rear of your lower receiver extension bridge.

Those marks on the top of your lower bridge are a lot like cylinder rings on revolvers. A lot of neat freaks and newbies think it's an indication that their firearm is misbehaving - but it's just an indication that it's functioning as designed. It's "character." Guys that buy cool basketball shoes might have white sneakers that stay bright white. Guys that actually play basketball might still buy white shoes, but you can tell that they actually PLAY because the sneakers tell the story.

Eventually your buffer tube will get tracks from your telescoping stock, your dust cover will leave a mark on your lower, and vice versa, your charging handle will get stripes worn down its sides and top, your upper will get a line worn where the charger latch rides, your mags will get streaks, your safety might even spin a ring on the side of your lower... Let alone if you're actually shooting the rifle and getting nicks and stratches and scrapes...