@sydsnyper, I'll add to Kunal's explanation.
The LCH was designed to be combat operable at very high altitudes. Understand that I mean combat operable means being able to do the entire envelop of maneuvers and tasks.
Many helicopters can reach the same (or nearly the same) altitude as the LCH, but not one is combat capable at that altitude. And that's the reason why you take the service ceiling of the Apaches and the lot with a bag of salt.
To enable the sterling high altitude performance, every extra kilo that could be shed without compromising fighting capability was shed. The choice of a chain gun (20mm) is a logical choice to that end. As Kunal explained, a gatling gun is much heavier than a chain gun.But that is not all,
a heavier gun requires a stronger and thus heavier support structure. Thus you cut two causes of high weight by going for a lighter single barrel gun.
And about 30mm chain guns, well the Apache has that. But why shouldn't you use it on the LCH? I'll show you.
The average weight of the M621 cannon on LCH is roughly 46 Kg. The 30mm M230 chain gun on AH64D weighs 56kg. So, a difference of 10 kg.
Not much, I should think. But then,
A single 20mm round will weigh approximately 100gm. For 400 rounds, this turns out to 40 kg.
On the other hand, a single 30mm HEI round will weigh 240gm. for 400 rounds, this turns out to be 96 kg.
So as you can see, a simple shift from 20mm to 30mm caibre will increase weight by 106kg, which is big considering the developers of LCH were fighting to reduce this much weight for quite sometime. .
