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The most likely calibers are 5.56 mm or 7,62 mm. 

How many yards away was the shooter? How does this affect the sound from a mic that was likely directional towards Trump? What's the quality of the audio feed as delivered to your TV?

CD, you almost as a rule refrain from idle speculation without hard detail. You *really* feel comfortable about this one?

You rang?

That isn't the report being heard... that is the ballistic crack made as a projectile passes a stationary object at supersonic speed. 

The caliber was less than a 30cal (7.72, 270, 7mm, ect) else the area of high pressure in front instantly/violently being replaced by an area of low pressure, aka percussion aka concussion in this regard would have blown out his eardrum and a sliced lobe would have been the least of his concerns- he swatted at his ear, if larger caliber he would have cupped his ear of not lost balance and fell as his drum ruptured.

The flags to the left of stage were less than 30 degrees angle and gusting to that from nothing, and towards 1 o'clock from trumps position.  That indicates wind speed at the target (or close enough to adjust for).  It would also have been a quarter value wind from the shooters position.  Full value (left to right or right to left) and zero value (into or with the wind) are easier to factor than quarter values which are tricky even for seasoned shooters.

Why is any of this important? .... glad you asked... it isn't now, as that shooter lost his head to a meat mist, but up until that happened that held extreme value for the defenders aka security detail... they'd know the shooters capabilities in terms of reach... they'd know who was better suited to engage from the open in terms of armor so they could "run the rabbit" which is to send someone in harms way to draw fire.  They'd know the skillset of their opponent(s).. they'd know their general distance and direction.

They'd still be looking for a trained sniper if that was the case, which fortunately it wasn't.  As it turns out they're reporting on sky it was an antifa clown- you know, one of those harmless types without reap organization. 

Trump was animated, and that helped him tremendously... and that was a cold bore shot and I bet something the shooter knows nothing about else this convo would be much darker. 

How could a shooter get a long rifle so close?

One report said AR15, which is obviously tentative.

I've seen ar platform... that could cover anything from a bb gun to a 50 cal.  

The ballistic crack gave it away.. I'd put money on a 5.56 aka 223 and the most popular 22cal other than a 22lr rim fire.  

It was a mostly peaceful assassination attempt, at least.

From the NYT:

 Officials also said that they had recovered an AR-15-type rifle near his body. Videos posted to social media and verified by The Times showed the suspected gunman lying motionless on the roof of a building around 400 feet north of the stage. In an interview with the BBC, a man said he saw somebody with a rifle on a rooftop before the shooting and tried to signal to the Secret Service.

Now, one can take this "AR-15" reference with a grain or three.  It's pretty obvious the rifle was semi-automatic, and the most common rifle that is would be 5.56 mm caliber (or 7.62, which makes a different sound, in my opinion even on TV).  We'll know soon enough.

As to how anyone with a long gun could get that close to a protectee .... well .... this will be a news item for a few weeks too.