You have No idea as to how many possible shots SA Hickey might have fired. Personally, I believe a shot fired from the AR-15 would have penetrated that chrome trim. Not dinged it. If it was a possible AR-15 bullet fragment, that fragment should have remained inside the Limo and then been recovered. A chrome strip of any kind is flimsy. It's not hardened steel or anything close to that.
I do believe there was damage done to that area below/(S) of the Shelter behind Zapruder. There's a concrete walkway that runs between that Shelter and The Steps. The TARP/TENT covering this same area that is visible on the SS recreation footage is proof of work being done on this area. Probably concrete work. We know for a FACT that the Black Dog Man Nook was modified in order for the short wall section to snugly intersect with The Steps. On 11/22/63 that short wall did NOT intersect with The Steps. Footage of people running UP The Steps after the assassination clearly shows a GAP between that short wall and The Steps. Today, there is NO GAP.
The chromed strip was probly stainless steel, very hard, & it had a crimp, & an air gap to the tubular steel windshield frame.
The hollow point AR15 0.223 slug hit the crimp first. The slug fragmented into smithereens, firstly mainly down (as per the crimp) damaging the rear view mirror. Then the small bits of lead splattered all round at 360deg, mainly backwards, making a large smooth crater, & probly no remnant slug, but in any case the remnant slug would have ended up in Elm St.
SSA Floyd Boring who inspected the AR15 after, said that an auto burst would consist of at least 2 or 3 shots. I have said that based on the evidence the auto burst was of at least 4 shots, probly 5, possibly 6.