I think this mission is badly designed. It's frustrating and pointlessly hard to do.
If you don't bring a sniper rifle from previous missions, there is just NO WAY to get the one on the guard tower, because the enemies will see you (no doubt) and swarm you. Also, there isn't a "clean" way to get inside the lab to place the bomb, and you WILL have to kill the guards outside (and being swarmed by the whole compound).
That just sucks...
You don't need the Sniper Rifle - the M60 will do the job nicely and the Guards won't mind you carrying it. There is a silenced pistol in one of the tents within the compound that will also be useful. You can clear the mansion of Guards safely if you knife them in a specific order. The ones outside the door is where you use the silenced pistol - after killing all of the interior Guards then open the door from the inside and shoot them in the back of the head. One of the Guards inside the Mansion wears a light khaki outfit which will let you safely inside the bunker so you can plant the bomb.
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