: That really doesn't apply a whole lot to large-caliber, anti-material rifles
: like Halo's sniper rifle. The vast majority of anti-personnel sniper
: rifles used today are medium-caliber rounds like .308, .338, etc. and
: don't sound like a Howitzer. Of course, all of these can be suppressed,
: but I'd imagine you would significantly deteriorate the speed, distance,
: and impact of the round fired with it.

This. I'm no expert, but I'm pretty sure Sniper Rifles (especially Halo's) are going to be stupid loud. Even suppressed. And it's not like it HAS to be quiet. It's a Sniper Rifle. The whole point is you're hitting something from REALLY far away. It doesn't matter if they hear you if they can't hit back.