Can you please let me know your ballistics or forensic experience to say for certain that its an airgun?
The heat required to turn steel orange is nearly 1000 degrees, no airgun can propel a projectile at sufficient speed to cause that much heat in compression (*), and you can see that its the primer / protection layer that is discoloured and not bare steel. The discolouration marks are material transference from the item used to cause the damage. The impact has been made by something with a hollow or recessed tip, maybe like a hollow point pellet, but a hollow point is designed to expand on impact which would have flattened out upon initial impact and not left the circular paint mark with raised lip.
I stand by my suggestion that its an umbrella tip or similar.
No experience in ballistic forensics but you can be assured that I know a bit about bullet impacts and their effect on things.
My firearms experience? (Because I just know you are going to ask.)
Comp pistol shooter for the West Midlands 1989 - 1992.
Pro pistol shooter for the UK 1994 - 1996.
Firearm demo and sales for H&K 1996- 1998.
Tactical Firearms instructor for Sig Sauer 1998 - 2001.
Security and firearms advisor for foreign private security firm 2001- retired.
Shotgun instructor 2005 - present.
Assistant firearms instructor to various UK police forces, Occasional 2005 - present.
UK PSG range officer and instructor 2009 - present.
Local gun club instructor trainer and range officer 2009 - present.
NRA range officer 2010 - present.
(*) A 250grn full metal jacket .45acp round fired at 1/2" steel plate from 25yrd generates compression heat, but not hot enough to burn your finger or discolour the steel in anyway, other than material transference. This I am certain about, I use these bullets twice a week.
Bullet travels around 750fps and has around 312 f/lbs of energy, a legal limit air rifle is 12 f/lbs, a pistol is 6 f/lbs.
So, in conclusion the bullet with 300 f/lbs more energy than a pellet cannot heat steel enough with energy compression to turn it orange.
Still think I am a ****?