The clip -- which is now circulating on social media -- shows the seconds after 20-year-old Thomas Matthew Crooks fired toward the stage Saturday in Butler, PA ... picking up as one of the bullets ricochets off the far railing of the stands and sends people scrambling.

Watch ... the noise from the shot rings out, and a small puff of smoke explodes into the air as the bullet strikes the metal rail. Two people standing nearby immediately react.

At the same time -- closer to the center of the stands -- a man on the top row drops when the gunshot goes off ... it almost looks like he might've been hit. Others around him also start to hit the ground in attempt to cover -- but the man in white goes down hard and fast.

Like we said, some online believe this man was hit by the bullet and injured ... but it's unclear if this is the case or if he simply crashed to the ground after hearing the noise.

Of course, we know one man -- Corey Comperatore -- was tragically killed ... and two others were injured ... not to mention Trump himself, whose ear was grazed by a bullet as well.

Video from news broadcasts that day seem to show just how far this bullet went ... with some suggesting it continued on its way and hit a hydraulic hose on a forklift holding up a speaker system ... with water gushing out right next to the stands where this happened.

In any case, it's another angle showing just how close others at the rally came to tragedy when Crooks fired off a series of shots before snipers promptly took him down.

Other videos have come out showing just how long TMC had to set up to get his near-fatal shot at the 45th President of the United States ... and it sounds like he was roaming the grounds -- on law enforcement's radar -- for a while before he got on the roof and fired away.