[Ulzgoroth]

For game mechanics, note that you will desperately need an extended Pi damage scale and target SM based modification of injury factors, or you'll get abject nonsense toward either end.

Scaling up, we already know that big guns work out pretty well. Big creatures are probably fairly happy with using larger calibers. They need them for stopping power against similarly huge targets, and they probably expect to engage at longer ranges than humans do so better velocity-retention is a plus. They might find automatic weapons tricky. Bigger, more powerful rounds usually cycle more slowly. It might not be feasible to build an assault rifle in 50-60mm.

Scaling down, if you could keep pressure equal you might be able to achieve equal velocities, but thinner construction means you can't keep pressure equal. Even with much shorter ranges you'll be seeing less proportional penetration than a bigger gun. (Of course, scaling down also makes melee combat almost unrecognizable, as most striking attacks are rendered ineffectual by the low mass of potential strikers. At ant size, throwing a punch would be useless. Wrestling-style continuous force tends to rule instead.)