The Arakan Army released a video in April of 2018 that was commemorating the 9th anniversary of the founding of the rebel group that shows a Barrett MRAD in a Multi-Role Brown Cerekote being fired during the promotional video.
Due to the multi-caliber options of the rifle, we can’t say for sure whether the rounds are .300 Win Mag or .338 Lapua. It appears that the rifle is mounted with a variable powered optic with an illuminated reticule and an objective lens parallax adjustment. The optic is most likely a Chinese product in addition to the scope mounts which appear to be of low-quality for a rifle shooting a heavier recoiling round such as the .338 Lapua or .300 Win Mag. There is a factory section of Picatinny Rail mounted to the 3 o’clock portion of the handguard indicating that the rifle might be relatively new as the rail hasn’t been moved since the factory.
Also of particular note is the compensator which isn’t standard on factory Barrett rifles on the commercial market in the United States. The compensator with the Arakan Army has 3 small ports in comparison with Barrett’s standard version that only has 2 large ports. Barrel length on the Arakan Army’s version is either a 24-inch barrel because both the .338 Lapua and .300 Win Mag have this 24-inch configuration available as opposed to the 20-inch barrel which would put close to the handguard and this isn’t the case in the video. Also note the Atas (or at least copies thereof) bipods.
As to how these MRADs made their way into the Southeast Asian country of Burma, it is most likely via India. These are most certainly not Chinese copies in some fashion either. The M82/M107 has been copied previously in Iran but the MRAD is relatively new and essentially unknown in many parts of the world. One route may have been through India which has been looking for new precision rifles since 2016 but has yet to field a replacement for the Dragonuv currently in service. Barrett might have provided a few submissions that might have been diverted to the Arakan Army once in Indian hands. It is known that Carl Gustav rocket launchers that were sold directly to India, end up in the hands of the Burmese Army, which then had them captured by the Kachin Independence Army (KIA). The KIA also happen to be very close allies with the Arakan Army.