The secret is short bursts. Long, sustained fire can jam almost everything.
Well, except possibly the various Maxims, but they where designed for sustained fire.
Short burst can jam too, it's just less likely. The larger size also makes it much harder and longer to clear jams. It also makes it much hotter and require a more robust cooling system. It's a pretty absurd weapon, even if it isn't really impossible.
It also should be, well, bigger than it is, and being inaccurate due to recoil effects on your APCs. 20mm autocannon are big mothers.
It's about liquid propellants, mostly. We use solid fuel. Like gunpowder.
As for the muzzlebrake, I direct you to the suspension system we spent, what, two actions on? That doesn't do too much either.
There's a very significant difference between deflagrating propellants and solid fuel rockets. You don't need an engine for the gunpowder, you don't need to worry about fuel flow, you don't need to worry about fuel stability, you only need to worry minimally about packaging.. some of them are issues to some degrees, but everything is ten times easier when you don't need it to actually explode continually. The two are not 'like' each other, not one bit.
I'm not really worried about the muzzlebrake. I'm miffed by being told that Moskurg's tank cannon was specially good when Arstotzka's very next turn involved designing the same gun, but bigger. It's just hypocritical.
Oh, and evilcherry, it's also because if someone jumps out and says 'flamethrower weapons suck', people still vote for them anyway.