ZM-
Not sure.
Deep inside the TSBD, three boom sounds were heard. That indicates three shots from the TSBD6 sniper window, IMHO. There could have been shots from elsewhere, and the second official investigation into the JFKA, the HSCA-1979, said there was a high probability of shot from the GK.
Despite the M-C carbine or short rifle firing supersonic bullets, no witnesses reported the cracking sound sometimes associated with such bullets.
I believe I have heard "crackle" sound of echoes off of trees and rocks from high-powered hunting rifles--the slugs leaving a miniature supersonic booms as they go back.
Bowers did not report a "crack" sound, only three booms. The last two booms very tightly spaced---the same recollection of Deputy Weitzman and also SS agent Kellerman.
I suppose people close enough to JFK and JBC would have heard the impact. Bowers was likely too far away.
Bower did not report one boom sounding different in pitch or volume from the other booms. Only the rapid cadence of the last two booms--which rules out the sole use of a single-shot-per-bolt-action-rifle as the source of those booms.