Thomas Matthew Crooks Botched Rifle Club Practice, Clearly Practiced Since

Thomas Matthew Crooks clearly put in serious practice at a gun range before trying to kill Donald Trump -- because now we know just how bad of a shot he was in high school, and why he didn't make his rifle team.

TMZ has spoken to sources who are directly familiar with Crooks' attempt to get on that team at Bethel Park High School, and we're told he had a disastrous effort on the shooting range ... sometime between 2018 and 2019 which resulted in him not making the squad .

Our sources -- who knew Crooks personally -- tell us he wasn't just a bad shot ... he was a total mess with a firearm, which was proven when he accidentally shot into a completely different lane from his own.

We're told Crooks was out at a practice session -- which was serving as tryouts -- and while he was trying to shoot at a target about 107 feet away, he ended up hitting someone else's target to the left of his.

Our sources, who saw him miss that badly, tell us this is why he didn't get a spot on the team.

It's also why another former classmate says they were surprised Crooks got as close to hitting Trump as he did -- he simply sucked that badly years ago.

What one can surmise from this is the guy obviously got better in the years since this botched practice, because he was within an inch of taking out the former prez.

Another source, also privy to Crooks' failed tryout, tells us he was nearly incapable of hitting any of the 10 targets on a 10x12 paper -- and at one point, he either had an accidental discharge downrange, or lost track of which target was his and sighted on someone else's.

There's also this ... we're told the other rifle team members would make fun of Crooks behind his back over his lack of skills, but no one ever witnessed him being outright bullied ... as some reports have claimed.

Notably, our sources all said he didn't display any aggression, and was simply a socially awkward kid.

DONALD TRUMP'S EAR INJURY

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As we reported ... the 20-year-old gunman was killed by the Secret Service last weekend ... but not before he squeezed off several shots from an assault rifle ... one of which hit Trump's ear, and another killed audience member Corey Comperatore .

We know Crooks frequented a gun range in the area, but as we reported ... he was never on anyone's radar there. No one really knew him.