You're fighting an uphill battle. Actually, you're fighting a battle you can't win.
Over the years there have been countless threads with countless ideas and reasons as to why MG fire should NOT be able to bust up a 4ft (or more) steel reinforced concrete bunker, but alas here we still are destorying ammo bunkers with a single pass of a P51's MG fire.
I've vouched since I've started playing the game back in Jan 2008 that having the same amount of dmg required to destory a town building, a fuel tank, an ammo bunker, a radar tower, and a plywood/canvass barracks is a bit (ok, WAAAAAY) over-simplified. I'm told the reason for that is to allow for the **extreme** most number of planes and gv's in the game to be able to get the job done.
Sure, valid point and point taken. But still.... I think there should be some measurable difference between barracks made of plywood and canvass, and a 4ft thick steel reinforced concrete bunker. The same goes for the giant church building and the 8x8 ft shack in the town.