This problem has and still is happening since launch, everytime in single player when I try and fir any shotgun or pistol, the game slows down and kind of stutters. I could blowup fifty things, and it will run smooth, but a simple shotgun or pistol blast makes it slow down?

i5-2500k
8gig ram
gtx560ti
1920x1080

In-game open the console then type r_PostMSAA = 0
Really, do this it helps, also after joining a server when you're in MP.
if you want to be adventurous, copy the following into game.cfg and delete what was there before or just try some of the settings, BUT after tweaking set the game.cfg to 'read only', which is right click > properties.
Also if you're using hdmi your refresh rate may be downscaling, you need to toggle in-game v sync on then off.

Paste this>>

-- [Game-Configuration]
-- Attention: This file is re-generated by the system! Editing is not recommended!

r_Driver = "DX11"
r_fullscreen = 1
r_Height = 1080
r_Width = 1920
sys_highrestex = 1
sys_spec = 4
g_skipIntro = 1
r_colorgrading = 4
r_DepthOfField = 0
cl_fov = 80
pl_movement.power_sprint_targetFov = 80
r_DrawNearFoV = 80
r_HDRLevel = 12
r_HDRBrightLevel = 0.3
r_UseEdgeAA = 0
r_TexMaxAnisotropy = 0
r_PostMSAAEdgeFilterNV = 0
r_PostMSAA = 0
r_SSAO = 0
e_GI = 0
r_HDRBloomMul = 0
g_radialBlur = 0
i_mouse_accel = 0
i_mouse_accel_max = 100
i_mouse_smooth = 0
cl_zoomToggle = 1
cl_crouchtoggle = 1
g_enableInitialLoginScreen = 0
r_MultiGPU = 1
g_useHitSoundFeedback = 1
r_NightVisionBrightLevel = 0
r_NightVisionCamMovNoiseAmount = 0
r_NightVisionCamMovNoiseBlendSpeed = 0