Inexplicable Wounds made by Special Bullets

[Bob Mady]

This should be interesting; CLINT HILL heard the first 'firecracker' like sound but never heard the second rifle shot, why is that?

CLINT HILL: SSA REPORT "I was visually scanning these people when I heard a noise similar to a
firecracker. The sound came from my right rear and I immediately moved my head
in that direction. In so doing, my eyes had to cross the Presidential automobile
and I saw the President hunch forward and then slump to his left. I jumped from
the Follow-up car and ran toward the Presidential automobile. I heard a second
firecracker type noise but it had a different sound-- like the sound of shooting
a revolver into something hard. I saw the President slump more toward his
left."

HILL starts moving to the limo after z-300 and does not board the limo until after rifle shot 3, that would mean HILL most likely did not detect shot number 1, but did detect shot number 2, to be like a 'firecracker' and reacted as soon as he saw the President was in distress.
What sort of suppressor can be that good but so specific?