Lie-La-Lie .. POW!

There comes a time in one's life when you start asking the questions that really matter - like how did they do that giant gunshot of a drum fill on "The Boxer"? Hal Blaine, session drum player for everything under the sun back then (and part of Phil Spector's "Wrecking Crew"), answers this in a Spectropop interview:

"Roy Halee would walk around clapping his hands looking for kind of an echo effect. And we were at Columbia in New York on the sixth floor I believe it was, and from the studio you kind of walked out and down and it went around almost like a ramp to the elevator. And he found a spot right in front of the elevator that had a tremendous echo and he loved it! This was a Sunday and we were doing 'The Boxer' and they had me set up... I set up two giant tom-toms right in front of the elevator where Ray had found the great echo. And of course there was a line coming out for my headset, so I was obviously the only one who could hear the music... (singing) lie la lie POW! lie la lie la lie la lie lie la lie POW! And at one point my hands came down to hit that smack and the elevator door opened and there was an elderly gentleman in a security guard uniform. And I guess he thought that he just got shot! It was like a shotgun, POW!"

Here are a couple similar stories about their improvised percussion sounds:

"We were mostly doing live tracks, maybe overdub a few things, and then put vocals on. But we'd experiment, too. Like on “Summer in the City” that explosion is from putting a mic in a garbage can that we had in the studio and banging it." - Roy Halee interview

This is presumably the faux-snare crash at the beginning of the song.

"That was just a matter of percussion sounds, coming up with different sounds. I remember playing my snow tire chains on "Bridge Over Troubled Water."" - Hal Blaine interview

It remains to be seen how one plays drums with snow tire chains, but other interviews describe them being beaten onto the floor of a storage closet. In any case, the result comes in towards the end of the song, and sounds like cannon fire.