[@Ollumhammersong]
The main thing is that being encumbered is not the end of the world. You get -10 on mobility related tests, -1 Agility Bonus for initiative and movement, and after 4 hours you have to make a toughness test (which you have nearly a 50/50 chance of passing) to avoid gaining a single fatigue point. After gaining 5 fatigue, you would fall unconscious. So, at the very least, you'd need to be lugging that gun around for 20 hours (assuming you failed every test). 

So, that means you shouldn't be swimming, climbing, or doing shit you really shouldn't be doing while carrying a machine gun anyways, and you shouldn't march for an entire day without stopping, which shouldn't happen anyways, heavy gunner or not. You have an 11% chance of passing out after 20 hours of carrying a 30kg gun and that's about the only penalty that should actually effect you. If you think that is too much of a problem, you can always choose a new role, nobody is forcing you to be the gunner.

The only thing heavy weapons impose a large penalty to is when you attempt to hip-fire them (which is a -30 BS and limiting to single shot) which you should not do. 

TLDR: Being encumbered shouldn't matter except in rare circumstances when a heavy weapon would be a bad idea to have anyways. So, as long as you aren't like 40kg over your limit, it's fine.