Silencers
Silencers were available for pretty much all weapons, but no silencer can stop the sound of the bullet breaking the sound barrier because the shock wave travels right along behind the bullet.
All a silencer really does is give the compressed gas a bigger area to expand, and therefore muffles the sound of the explosion. But the bullet itself makes one heck of a crack as it breaks the sound barrier, so while a silencer helps, it's really only effective if the bullet is slower than the speed of sound. This means very limited range, and very little power.
Generally speaking, a silencer just isn't needed or wanted on a sniper rifle. Range and accuracy are more important, and you're probably firing from several hundred yards.
But a good silencer really has little effect on range or accuracy, and can help hide the source of the shot. The main reason silencers weren't used often was simply because they're big, cumbersome, and even with a silencer, a sniper doesn't stick around for long.