[Alastor Mobius Toth]

Fair enough, they don't feel pain. Or at least, the pain as we define it.

BETAs are not hard to hit, at least, as long as you have a halfway decent fire-control system. In fact, quite the opposite, individual BETAs are hopelessly outclassed in mid-ranged combat (laserhax notwithstanding). They are mostly slow and unmanuverable, while those that are fast (Tank-class), are still easy to kill if you manage to keep them at range. Their very numbers work also against them - a single BETA wave will have so many creatures in it, that even if you miss, you'll hit something else anyway.

Also, ammunition is less of a concern for a TSF (Which is a standard for almost every military sci-fi ever); an average TSF is supposed to carry a dozen or so of magazines, each which has 2000 36mm caseless rounds. And several 6-shot 120mm magazines.

The problem is that this ammo will run out anyway at some point. Burst/Semi-automatic fire will only delay the inevitable, or may even hasten it, as the BETA will overrun the troops that are not firing enough bullets.