I'll try to post some pictures later today regarding shooting in the vert, Owlblink and Hungry, anyone who's interested. Kingpin is comprehensively covering harmonisation / convergence I think. About aiming under or holding low - just think of a plane with nose mounted armament to remove the lateral spacing aspects. Indeed in a K-4 with a convergence of 600 (which I gathered from Bustr once was the actual effectively the fixed value in the real aircraft) you will have to hold noticeably low at 2-300 yards otherwise the shot will arch over where your pepper is pointing so it would hit the pepper at 600 yards. This assumes where firing pretty well in the flat.
I think why I noticed no difference between armaments with my quick and dirty testing is that in the vert, the bullet drop as we perceive it - or rather distance travelled over time is the same for all projectiles, velocities and shapes: 50 cals, 30-mm, 20-mm, wheel nut off a 1950 Volkswagen Beetle, what-have-you. Gravity is still there but it's now pointing downwards to the gun barrel it came out of. But the sights are designed to cope with you shooting when flying flat. I'm fairly sure it's one of those things where at an incline of 45 degrees it'll be 0.707 of the drop it is in the flat. Does that make any sense?
There was a time in the Ki-84 when I used to fire all guns at once and I had my cowl-mounted MGs set to 150 and my cannon set to 600. At the time an awful lot of my shots where at an opponent crossing my wing line in planform so I wanted to make the dispersion as 'flat' as possible to stitch down the centreline. Here the 150 convergence of the cowl guns weren't 'lofting the shot' up to reach a harmonisation point at a farther distance on my gunsight and the wing-mounted guns, which are physically lower in the aircraft were. I'm fairly sure I tested this with the dot target thingamajig. I was quite scientific in those days (which can take you to certain points of development).
It's all very interesting but since I've got back I can't hit a barn door with my plane parked inside it, pointing at the doors with the doors closed! I believe H. J. Marseille, for instance, relied more on the kind of 'software' we use for throwing and catching balls. That takes a lot of practice to form the software and a lesser amount probably, to keep it up. So it's a perishable skill. Apparently