I've finally swapped the texture on a gun after referring back to the old Gta 4 methods. It doesn't look the best, but it works pretty well.

Installation:
Make backup of your w_sr_sniperrifle.dds before starting.
1.Launch OpenIV
2.Navigate to: \update\x64\dlcpacks\patchday3ng\dlc.rpf\x64\models\cdimages\weapons.rpf
3.Once there, enable edit mode, open w_sr_sniperrifle+hi.ytd with the OpenIV texture viewer, and replace the w_sr_sniperrifle.dds with the one you downloaded.
4. Same process as step 3: in that same folder (weapons.rpf) open w_sr_sniperrifle.ytd with texture viewer, and replace the w_sr_sniperrifle.dds with mine.
5. Finish and launch.