Periscopes prove flat earth:

59 seconds
https://twitter.com/TNTJohn1717/status/1578424135029604358

They can see 16 miles!

US NAVY RAIL GUN HITS TARGET 100 MILES AWAY PROVES FLAT EARTH

https://www.bitchute.com/video/JxvyHIUpPTGQ/

https://flatearth.ws/railgun
Railgun

A railgun is an experimental weapon that uses electromagnetic force to launch high-velocity projectiles. Some railguns are expected to have a range of more than 200 miles.

Flat-Earthers claim that a railgun round always travels straight. And because it can hit targets beyond 200 miles, they use it as “proof” Earth’s curvature does not exist. They are wrong. In reality, railgun rounds are projectiles. The same law of physics that applies to bullets, arrows, or thrown rocks also applies to them. The rounds are affected by air resistance and Earth’s gravity, and will not travel straight for very long. Railguns are capable of hitting targets obstructed by Earth’s curvature and are not proof that Earth’s curvature does not exist.

Flat-Earthers probably took the conclusion after watching some videos showing that railgun rounds travel straight. However, the videos only show us the trajectory of a railgun round over a very short distance. It looks straight just like a handgun shoots practically straight over 10 ft. Over a longer range, the curved trajectory will be apparent, be it a thrown rocks or hypervelocity railgun rounds.
There are several docuмents from the US Navy that clearly show us that railgun rounds are not fired straight at the target but fired at an angle, with a parabolic trajectory, consistent with other applications of ballistics.

Presentations
From “Naval Railgun Tech Assessment” — US Navy, 2004 

Oh my goodness, more lies suited for liberal minds. This is a ridiculous attempt to reassign the laws of high velocity projectiles in order to pretend earth is a globe.  If this were true, how would the railgun account for gravity changes in differences in the height of land masses that aren't consistent for the entire range?  How could high velocity be maintained in an arc with changing gravity influences along the way if the velocity isn't the ruler? A high velocity projectile is not affected because inertia overcomes change in direction until it doesn't. The only way a rail gun can work is in a direct line. High speed trajectories don't curve, except perhaps, at the end, when they fall off from diminishing energy. Even then, that is minimal and calculable for the range, if they hope to hit the target.  Same with a gun.  Whoever wrote this is a nincompoop.  As if one anonymous opinion can somehow overcome thousands of years of the Fathers of the Church, as well as scripture and scientific proofs to the contrary just because people can't handle the truth.            

Geocentrism:  earth is stationary, shaped like a globe, and the vast universe revolves around it 30 (35.3%)

Flat Earth:  earth is stationary, the surface we live on is flat, covered by a physical firmament, and the universe is closer than we're told

26 (30.6%)

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So, I quickly Googled "how far can a periscope see".  Why is it that the FIRST search result at the top is a link to an FE debunking article, an article that doesn't even contain the phrase "how far can a periscope see"?  There's no mention of FE in my search terms.

It's true that anything taller than about 120 feet can be seen from 16 miles out, at least the uppser part of it.  So we'd have to look into what they can see from that far away.

There's one former military-submarine guy who's a Flat Earther who gave an interview where he explained a lot about how thy can see too far and whom provided a lot of detail, but I can't remember them offhad.

I'm not so sure after I read this from Haydock:


And God said: Let there be lights made in the firmament of heaven, to divide the day and the night, and let them be for signs, and for seasons, and for days and years: - Genesis 1:14

The day is completed in twenty-four hours, during which space the earth moves round its axis, and exposes successively different parts of its surface to the sun. It goes at a rate of fifty-eight thousand miles an hour, and completes its orbit in the course of a year. (Haydock)

- George Leo Haydock

Haydock was deluded like the rest of us before we realized we were lied to.  Incidentally, his numbers are way off.  According to NASA the imaginary globe travels quite a bit faster at 67,000 mph to complete a year around the sun which is almost 10,000 mph difference. So it's not like Haydock knew anything, he was repeating what he'd heard at the time.  That's not science.  It's also not teaching.  It is a lie planted purposefully by evil doers so error can be propagated by good people unintentionally, which is how the enemy operates, sowing tares.