Depends on the caliber, the amount and type of powder, and the bullet weight and what the camera settings are also. He said it is a construction site so I'm assuming there's a lot of noise coming from that area that could mask the sound of the gun being shot. plus there's makeshift silencers I seen on youtube just by putting a oil filter over the barrel.
You're correct there are a myriad of factors at play. I simply chose the most commonly sold round in the USA to base some simple math off of.
Something like a 175gr match .308 Winchester round is still traveling around 2650fps. That's only roughly 5% slower, not enough to effect the math to a large degree.
Regarding "makeshift silencers / IE Oil filters", the performance is laughable in comparison to an actual suppressor.
Suppressed firearms are anything but silent. To give you a quick run down, they're essentially designed not to exceed OSHA safe hearing levels (140db) in order to prevent hearing loss.
Most high velocity rifle rounds are brought down from 170-175db range to the 135-140db range, which is hardly "silent".
I can't take any of mine out in a metro neighborhood and start shooting without attracting some serious attention.
Bullets are actually relatively easy to spot in flight. To be brief, they create a vortex when flying through the air, which you'll hear most distance shooters refer to as "trace".
This is valuable data when making corrections when shooting great distances (500+ yards) as it allows you to "walk" your rounds onto target.
Even then, atmospheric conditions have to be just right, and you're not going to see it when looking through low quality glass.
Just sharing some experiences as a competitive 1000 yard shooter for a few decades.
Here's a short example on Youtube.
Notice the lack of trace elements in the original video, and that the original example looks nothing like a bullet in flight in the example here.
Start at 0:26