Can Ghouls be killed with bullets? In the last episode of the first season a lot of special forces with machine guns attacked the Aogiri Tree. My question is, given the regenerative abilities Ghouls have, can  they be killed with machine guns? If they can't be killed, what's the point of using them? And if they can be killed, what's the point of using Quinques if you can kill Ghouls with long range weapons?
Regular bullets can't pierce most Kagune, but there are special bullets used by CCG which are more effective. Q_Bullets, or Bullets coated with a thin layer of quinque steel, are enhanced bullets which do deal damage to Ghouls.

That being said, stronger ghouls are generally resistant to even those, as we see the top tier S and SS rated ghouls tank the bullets. Weaker ghouls are at risk of death as much as a bullet to a regular human is if they don't have good regenerative powers. You can see that best in chapter 60, which in the anime is 10-11 minutes into episode 11 of season 1, as the last few pages have Juuzo use Marudes bike to jump into a ghoul infested building, and he uses a machine gun to punch dozens of holes into a couple of ghouls heads, even showing them off in the manga.