I think you guys are overestimating the medium caliber AA guns.
1) it fires a shell somewhere around 1/5th as large as the 5" and those take pretty close hits to kill planes
2) you'll have to set the fuze to burst at a certian distance (only U.S. AA guns had proxy fuzes)
3) litteraly ONE pass by a D3A and this thing should be dead (gunners die = the gun dies)