One shift in the 3rd edition (N3) of infinity is modifiers to weapon ranges are much more focused. Giving many weapons a bad, normal, good, and best range. Some are even more cut and dry with just good and bad. It may seem to go without saying that knowing you weapons "sweet spot" is a huge advantage.
Knowing a weapons "sweet spot" is especially critical in a game like infinity where premesuring is not allowed. Getting to know your weapon profiles is every as important as troop profiles. Try mesuring out and studying the distances. Try laying out a table and a few troops, try and guess the distances then mesure and see how close you were.
One thing that helps me make snap distance calls is keeping in mind the length and width of the play area. If you know the length of the table is 4' its easier to judge what would appear to be half, quarter, or possibly a third of that distance in inches.
Know when to switch weapons, sometimes the HMG is not the best option. When things get close up pull that pistol and use it. Know the sweet spot for each weapon your guys carry. Each troop type has a tool box of options. Use them.