MosinNagant : 
https://sketchfab.com/3d-models/mosin-nagant-m91-55c00242c4024307a761a89167e39a7c

M1 Garand / EBR M1 :
https://sketchfab.com/3d-models/m1-garand-4dac51f2da3642d1b66838eac3126cdd
COD MW2019

STG44 N :
https://sketchfab.com/3d-models/stg-44-sturmgewehr-fa37bef729e141a6a29bb022a3e0be41
https://sketchfab.com/3d-models/stg-44-sturmgewehr-fa37bef729e141a6a29bb022a3e0be41

Enfield Rifle :
DoD model

PSG1 :
https://sketchfab.com/3d-models/hk-psg-1-sniper-rifle-ce70ca34a2584dc0a2444e39fe20b684

SVU : 
https://sketchfab.com/3d-models/svd-dragunov-b18837e316304521a5d2da8d258b6f25

M110 SASS : 
https://www.cgtrader.com/3d-models/military/gun/m110-sass-44757562-093d-49fe-83b0-75ec979df5ee

RPD MG :
COD BLACK OPS COLD WAR

MG34 :
COD MW2019


and many others for additional parts