Guys, you have a car right? if I shoot a .22 at the front of your car I will hit your radiator. You will be pulled over to the side of the road in a cloud of steam eventually. Depending on how big a hole it makes and how fast your driving.
Radiators are mostly built of soft copper & brass.They rip, tear, and make quite a hole when shot by even a small bullet. I shudder to think what a .50 cal would do to one.
Now lets take it one step further, instead of being mounted IN your car, its hung below it in a air scoup. Well airscoups woulld also scoup bullets.
Also as SOON as there is an airpocket above the outflow hose, you won't get any more cooling water to the engine. Because the water has to go UP to get to the engine. As soon as there is a leak it won't siphon.
Where in your care the hoses are at the bottom of the radiator to feed that engine water as long as possible.
There is no way to armour plate a radiator & still have it work. Same for self sealing. Thats why raidial engines are SO much more dependable at Air to ground attack.
No radiator to take a single bullet or shrapnel hit & bring you down.