Donald Trump was grazed by a bullet during the assassination attempt ... at least that's what a ballistic expert gleans from the way the former president's injured ear is healing.

Leonard Romero -- a forensic firearms examiner who's worked alongside the LAPD and several other law enforcement agencies in this field -- tells TMZ … Trump's ear healing so quickly isn't evidence he wasn't hit (as some have suggested), but on the contrary ... it's actually consistent with a graze from a bullet, as opposed to a more direct impact.

The way Leonard, explains it ... a graze only impacts the skin's surface -- it's more like a scrape or a scratch, and doesn't cause heavy damage, even when the bullet is from an AR-15 assault rifle, as was the case here.

Trump was bleeding from the ear after Thomas Matthew Crooks squeezed off shots at him during his July 13 rally in Pennsylvania ... then he wore a massive bandage to the ensuing RNC, where he accepted the GOP presidential nomination, and has since ditched the bandage altogether ... and his ear has no major scarring viewable to the naked eye.

FBI director Christopher Wray at first said there was some question whether or not Trump was struck by a bullet ... but the FBI ultimately backtracked and said, "What struck former President Trump in the ear was a bullet, whether whole or fragmented into smaller pieces, fired from the deceased subject's rifle."

Leonard says video from the shooting gives no indication Trump was grazed by a bullet fragment ... instead, he tells us he's confident the bullet grazed Trump's ear before hitting anything else ... and doesn't believe Trump was cut by broken glass from a teleprompter.

Of course, we'd heard from someone plugged into the Secret Service that he possibly was cut by glass -- or some other material that ricocheted off the bullet -- but Romero doesn't seem to agree.

In any case -- it sounds like Trump got super lucky here ... Leonard says a .223 caliber bullet like the one the shooter used causes devastating wounds on direct impact.

Again, Trump's ear is still intact and not scarred or mangled ... which backs up what Leonard's saying here.