But lusche, the 103 has standard range (around 1k) I thought.
That would put you within the danger zone for a really good buff pilot.
Having the 50mm ensures you never have to take that risk.
I would question the versatility of it as well, I only ever use 103's when I want to kill fighters as it only seems to be good for that.
Bomber/raider killing at low alt, the extra 4*20mm I find is much better because of the sheer amount of lead thrown at them.
The moment the icon changes to 1.0, you are at 1250 yards and can fire away. At that range it's way easier for you to hit the bomber due to the very flat trajectory of the 103. The bomber gunner has the problem that due to the 500yds convergence setting, only a tiny fraction of his guns will be on target (mostly just the 2 MG of the station hes sitting in) and his bullets will have lost a lot of their destructive power (B-29 20mm gun aside). The Mk 103 shells are tightly grouped and have lost almost nothing of their destructive power.
You are simply pwning every heavy bomber but the 29 in firepower.
For a "really good" bomber pilot it's easier to avoid 1.5-2k 50mm rounds coming in in single shots by clever maneuvering, than to avoid that hail of 30mm bullets. If he's really clever, he will slam on the brakes in to get the 410 closer to him as quick as possible, but that is very much against the instinct of most buff pilots, as they usually try (for good reason) to keep heavily armed fighter at bay.
Against buffs, the extra 4x20mm pale in comparison to the Mk 103. Much less effective range, shorter trajectory, less destructive power. They would only be a reasonable choice against fast moving and maneuvering targets like fighters.