It's not impossible, I know that. But the odds of the RNG actually spawning a tiny room full of 10 or more shotgun guys with a megasphere are extremely slim. You may as well call it impossible. The reason I said it was impossible is because it was easier to type than "(see: 0.001% chance)".
The chaingun isn't luck-based though. Generally when someone starts a new run they decide between shotguns and rapidfire weapons (unless they're doing some quirky pistol/melee build, you get the idea). On the harder difficulties you can get a chaingun on level 2 without even doing Hell's Arena, but what does the shotgun guy get on harder difficulties? Just a combat shotgun, sometimes not even that. I know I've done quite a few shotgun-only attempts on HMP and UV that have failed miserably simply because the RNG was not generous enough to give me a combat shotgun on one of the first few (~5-ish) levels.
What I'm getting at is... the combat shotgun and double shotgun don't have to be dropped randomly by an enemy, it's just an idea. However, they should definitely be easier to obtain.
Oh yeah, almost forgot. In Doom II, you find a double shotgun before you find a chaingun. In fact, you don't get the chaingun until a few levels later.