Material found inside of a car would not burn hot enough to melt the copper jacket from a bullet or steel from a gun. The copper jacket may be deformed from the fire, but it wouldn't be melted into a lump of metal. The fire would be hot enough to melt lead from a bullet, which would have melted into a puddle and then solidified once the temperature from the fire reduced below the melting point of the lead.
Melting points of each metal:
https://www.onlinemetals.com/meltpt.cfm
2500 F - Steel
1981 F - Copper
621 F - Lead
Carpet, plastic and other flammable material found in a car burn at around 1500 F
https://www.usfa.fema.gov/downloads/pdf/publications/fa-243.pdf
For those of you that followed the Teresa Halbach case, (Making a Murder), it was determined that skull fragments found in the burn pit had a hole caused by a gun shot. These skull fragments were tested and metal from a bullet fragment was "supposedly" found around the opening or hole caused by the bullet. There were things burned in the fire pit which contained metal such as a seat from a van and tires to name a few. These was no mention of these other things in the fire contributing metal to the bones in that case.