Having sandbagged and designed real life ammo bunkers in the army, it would take more than a n00b strafing with 20m or 50cal to take out the ammo bunker. Couple of reasons:
a) We dug it in. If we had time, a tractor with a backhoe or a front-end grader would dig out a 4-6 foot deep trench.
b) We sand-bagged the hell out of it - inside and out. We would sandbag primarily the top with re-enforced bars and poles - perhaps 5-8 sandbags high and then around 6 deep on the sides.
c) It was designed to collapse upon itself if the ceiling was hit. Sandbags would then fall on top of all the boxes of rounds and other ammunition to ensure no fire would set off other rounds.
d) Ammo was placed around the sides and under the ground level.
What would you need to take it out? Lots of Napalm, or several well-placed shells from a tank through the front door.
Airfields that I worked at had established ammo bunkers were re-enforced concrete structures with 10-20ft of rebared concrete. You would need a bunker-buster bomb to take it out, or a really really big bomb (Stuka?) or again, Napalm through the front door and down the stairs. There would be an elevator to lift up the ordinance - but that would be down the hall. Again, if it's an established bunker, hard to take out even in modern standards but not impossible. BTW - these were bunkers built in the 40's.
Could you strafe it? About as effective as peeing into the wind.
I would guess that two well placed 1000lb bombs would be enough to render/destroy the bunker. If you then dropped incindinary on the hit bunker, you should be able to watch the fire cook off the ammo inside. Now, that would be cool.
