My bet is now pretty much as well that a bullet that hit Kirk was causing both the shirt blow up (at least partially caused by the necklace he was wearing) and the wound on his Neck. Who shot it is another question. Of course it could be Tyler but also that it was someone else.
One of the biggest problems I had with the shirt behavior is that it appears to happen before the wound on the neck visibly appears but I think it is perfectly possible that the initial hit of the bullet isn’t visible because the entry and/or initial hit of such a bullet is so small and hard to see/catch by a camera and that it takes some time for the bullet to, first, cut through the flesh/bones, second, release its energy into the surrounding area, third release energy into the wider body and fourth cause the extreme damage to his neck.
I think it is possible and even likely that the bullet caused the behavior of the necklace and/or his shirt at any point in time between the first contact with the body and the point in time LATER that the wound visibly appears on the body and camera footage.