Apparently different noise reduction within a weapon category (Pistols, Shotguns, SMGs, ARs, LMGs and Sniper Rifles) have no impact at all on notice distance. Each weapon category have fixed notice distances, independent of displayed noise reduction. I have not tested all weapons within all categories but it have been consistent with the weapons I tested. MP5 for example (which have one of the highest noise reduction of all SMGs) have the exact same notice distance as the SMGs that have lowest noise reduction.
When a tango is within notice distance his over head UI icon will start to flash. He will not set of an alarm but he will be on high alert as well as moving and looking towards the general area where the noise came from. After a few seconds he will stop and the over head UI icon will no longer flash. Everything will go back to normal. You can use this tactically to your advantage (in same ways as you could use a noise drone, a decoy or a flare gun).
You can also rush a tango from behind and either press E to grab him and then T to knock him down or T to knock him down directly. Only way to move the body is to grab him while he is alive and walk him over to the place where you want to dump his corpse. Tangos will start the alarm if they encounter a dead body.
Safe distance (with silencer / sound suppressor):
- Pistol 20m
- Shotgun N/A
- SMG 25m
- AR 35m
- LMG N/A
- Sniper 50m
Safe distance (without silencer):
- Pistol 35m (iirc, have to verify when i get home to my notes)
- Shotgun 50m
- SMG 60m
- AR 90m
- LMG 110
- Sniper 130m
Yes, tangos will never react to sound you if you fire your unsilenced cal .50" sniper rifle as long as you are further away than 130 meters from anyone that can hear you. All tangos within 50 meters will react if you fire a silenced sniper rifle (during stealth runs – shift to a silenced pistol or SMG when you enter the base).