Waka Flocka Flame -- Loaded Gun that Triggered Arrest ... Thanks for the PIC, TSA!

Take a look at the gun Waka Flocka Flame sent through an airport scanner with 29 rounds PLUS one in the chamber -- posted by TSA's Instagram ... yes, TSA has an Instagram.

Shortly after Waka's arrest Friday the agency posted a pic of the gun/ammo ... adding the whole thing would have been A-OK had the owner just checked it as baggage.

The post doesn't name Waka ... but as far as we know, no other firearm arrests were made that day.

As we previously reported ... Waka's lawyer says his client accidentally grabbed his fiancee's bag at home with the gun tucked inside -- all just a big mistake.

As they say ... tell it to the judge.