(You need OpenIV in order to access the game files)

Drag 'n drop here:

C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\mods\update\x64\dlcpacks\patchday8ng\dlc.rpf\x64\models\cdimages\weapons.rpf\

To make the m249 equipped with the m145 work you will need to edit weapons.meta located in

C:\Program Files (x86)\Steam\steamapps\common\Grand Theft Auto V\mods\update\update.rpf\common\data\ai

once you right click and edit the file (OpenIV latest update includes a text editor) search for "w_mg_combatmg" (line 7038)
then look for "FirstPersonScopeAttachmentOffset" (line 7487) and change the value of 'z' to -0.0500