It's technically impossible for any assault rifle to be silenced. The bullet is moving so fast it makes a sonic crack as soon as it leaves the barrel.
To give you an idea: Imagine a every piece of cloth in your house simultaneously ripping in half in a very small fraction of a second. Then, as the bullet goes down range, it sounds like an small airplane as the sound wave bounces around the area.
You don't get this effect with unsilenced weapons because the sound is drwoned out by the bang.
I guess they didnt wanna stretch the truth too much by puttin silencers on sniper rifles. A suppresor for the flash would be nice though.