Though this "theory" has no Proof, neither does the SBT. Yet the SBT is routinely embraced as being legit. The SA Hickey accidental firing of the AR-15 is certainly not "silly". I don't believe the AR-15 was responsible for the JFK Head Explosion, but an accidental missed shot(s) fired from that gun is very possible. This accidental firing would explain the smell of gunpowder stretching down Elm St as this fired weapon inside the Queen Mary traveled down Elm St and then onto the Stemmons Fwy On-Ramp.
Did the AR-15 have a silencer? None of the other agents nor David Powers heard a shot fired from inside the Queen Mary. No one could miss hearing an AR-15 fired from just a yard or two away.
A weapon was fired on Elm street, but smell by GK was from a distraction fire cracker tossed to distract from real shooters. Smoke, noise but no weapon.