Not to mention, the Bofors 40mm of WW2 was not really a machine gun, but a high-rate of fire cannon (about 2 shots per second) and used explosive shells just like the FLAK 88. Those big black clouds one sees erupting in the air against aircraft in WW2 movies were all based on those principles. The same principles were also used with the 5" guns on destroyers of that era as well, though they fired a much lower rate of fire at the time.
But as aircraft sped up and could start hitting from beyond visual range, they were slowly phased out, though some still exist here and there and can still be useful against NOE craft like helicopters.
So, yeah, FLAK ammo on Rifles would make sense, but they would be limited to what would basically be on the map itself. Not really touching the altitude map with the aerospace fighters, but really good at taking out VTOLs.
"effective'"? think about your comparisons here.
WW2 (and mainly pre-1937 designs at that) aircraft (like the Mitusbishi A6M "year Zero" fighter) were thin-skinned, slow-moving (less than 500 mph in level flight at TOP speed) machines. automatic cannon fire has proven "less than effective" against even very OLD jets in the modern age, and single cannon fire went out in the 1940s to 1950s. (They weren't trying to use 103mm long-tube guns to shoot down those B-52s over Hanoi for a reason.)
"rifle cannons" are not even self-loading weapons, much less rapid-fire self-loading weapons that can track in on a target and bracket it.
It would be like trying to shoot skeet with a bolt-action single-shot, or a martini-greener, only the skeet is coming in fast-moving pairs and missing means getting hit with 500 pound bombs.
On the 5" Dual Purpose guns: They were effective mainly because the horizon is far away, the enemy has to get within easy range, and you have a long time to aim. (you know, they're flying at 5000 feet over a perfectly flat surface with no obscuring terrain to disrupt the shot, slow enough that your crews can load and fire more than a single round...manually.)
Flak guns made lots of dangerous smoke, and did chip away at thin-skinned, unarmored, lightly built aircraft running between 200 and 400 miles an hour at 20,000 feet, in a straight line, in large formations. They did...less well...against low-flying aircraft like Tomahawks over North Africa, or those bastard soviet birds with the heavy protection on the Eastern Front.
but in either case, they were ONLY effective when used in large numbers at obvious targets. One of the reasons for the Schilka or similar AA machinegun mounts, is that they can put up a cloud very quickly, and track in on a target using fire. (also one of the reasons the Partisan AA tank in game has FOUR Autocannon 5's)
we're talking analog of a TANK cannon here-as in the modern ones, like the 105mm or 120mm Rhinemetall, firing a single shot from each pull of the trigger before needing to reload/have the loader load it/use the swing arm to load it, not a gun that fires like a bofors, with 2 to 5 round bursts.
Proportionally, FLAK ammo would be just about useless against a flying or fast-moving target, though it might work fine against a Yellowjacket, since that thing flies slower than most automobiles drive. (60 KPH isn't very fast, and 90 is slower than most freeway drivers will drive).
per the conversion formula, a Yellowjacket's TOP SPEED, is 55 miles an hour. That's going balls-out, hell bent for leather, risking violent sideslip, 55 miles an hour.
There were light observer planes in WW2 that were faster, and that's prop-driven taildraggers.
