The Engine is your best bet based on what we learned last year.
Here's some first hand feedback when we took Boomer (.50 Barrett Semi) and .308 and .223 to finish off a 1998 Volvo 940 Sedan near Austin TX last year. The 940 had taken shots from #6-8 birdshot, buckshot, .223, 308 and then Boomer. Knowing who had previously shot what and from where made this pretty easy.
Everything broke the windows, some more dramatically(Buckshot) than others.
#6-8 birdshot generally didn't penetrate the side panels from 50 yards.
Buckshot generally went through the side panels but not both sides of the car.
.223 Ball generally went through both sides of the car doors unless it hit something inside, like the window crank.
.308 Ball went thru the doors and objects inside them easily, and put nice non-penetrating dents in the engine block or components nearby, and would dent the brake rotors. It also did a pretty nice job on the wheels. The .308's also did several nice "front grill -radiator - non-structural stuff in engine compartment - thru firewall - thru car seats (front and back) thru back of trunk and out. Also went through the tires pretty easily.
.50 BMG Ball put 1-3" holes in the engine block, were generally distorted and didn't come out the other side, or busting through the aluminum wheels.
.50 BMG AP / API went through both sides of the engine and out the other side of the car except when it hit something hard like the crank, then it just bent or was wedged in tight. the API rounds had no problem busting or going thru the brake rotors.
Most of these rounds have much more energy than 9mm or .45's, so I'll let you do the math, but suffice to say, car sheet metal is much thinner today than in 1956...