Gareth Boll

I just started reading this book and it's great. But the one thing that irritates me is the fact about the encounter the character had with the silencer. He/she/it shot her in the leg with a high powered snipre rifle... First of all, they are usually so strong you'll loose your leg/ die from the shockwave. And the fact that the sniper probably killed the 3 people with headshots, hence a sniper being patient and not moving. Why didn't he kill an armed person with a headshot?