That's possible.... But it seems more likely that JFK was hit by a custom bullet.....( not custom caliber)
It seems to me that the bullet disintegrated and dissolved inside JFK's skull. One substance that immediately comes to mind is mercury...however a projectile filled with water might also have caused the massive damage.....
I do Not believe the Hickey AR-15 issue has thoroughly been researched and discussed in general or specifically on this Forum. The AR-15 is usually immediately dismissed as out-of-hand. The discussion on another thread on this forum regarding the smell of gun powder inside Dealey Plaza would easily be explained by the AR-15 having been fired and then put in motion inside the Queen Mary as it traveled down Elm St and then onto the Stemmons Fwy On-Ramp. The only other option explaining gun powder being smelled: (1) around the TSBD, (2) down Elm St, (3) in the parking lot behind the picket fence, and (4) on the railroad bridge running across the Stemmons Fwy On-Ramp = Many shooters. I believe that the gun powder odor being smelled by the DPD cop positioned on that railroad bridge, (which is a good distance from Dealey Plaza), would point toward the gunpowder odor following along with the JFK Motorcade = The Queen Mary. The Hickey issue with the gunpowder odor also in mind merits being completely fleshed out.