The FX-05 Xiuhcóatl is a Mexican-designed assault rifle. Named after the “fire serpent” from Nahuatl mythology, it fires the 5.56×45 mm NATO round and was created to modernize Mexico’s military small arms, replacing older 7.62 mm rifles like the HK G3. Looks almost like a G36, feels like a G36, but apparently HK tried to sue them but couldn't find a real reason to sue them. So in the end, it's probably just one of those externally copy-cat like rifle, as internally it is slightly different enough that HK drop the law suit.
Though the rifle probably takes its own proprietary magazine, let's assume that they do share commonality that it can be shared. It has a rail at the top to accomodate for optics, as well as side and hidden bottom rail. The stock can be pulled back and folded aside too.
"FX-05 Xiuhcoat Assault Rifle" (https://skfb.ly/ptIFK) by Martini-SF is licensed under Creative Commons Attribution-NonCommercial (http://creativecommons.org/licenses/by-nc/4.0/).
Thank you Riceler for the support!