In openIV open update\x64\dlcpacks\patchday3ng\dlc.rpf\x64\models\cdimages\weapons.rpf
drag and drop files into this folder and delete w_pi_pistol_hi+.ytd

For sounds:
In openIV go to x64\audio\sfx\RESIDENT.rpf
Export weapons.awc into openformats
Drop 0x0F4DFCD5.wav from right channel into weapons folder
Import weapons.oac back into RESIDENT.rpf

After that go to x64\audio\sfx\WEAPONS_PLAYER.rpf and drop ptl_pistol.oac from "left channel" folder into openIV

Enjoy