This drives me crazy, I can attest that the TAC-50 McMillian (C-15) is .50cal (12.7mm). The same is true with the Barrett and any other BMG class included in this game. In addition, it is not only the ammunition, it is the IRL pro's and con's of deploying a BMG given their size and weight. You do not fire a BMG from the standing position, or get into CQC with this monster.
In Canada we also use the C-14 Timberwolf MRSWS that fires the .338LM, which is a gem to fire, but less range and higher mobility given the lower weight. The C-15 is used for specific missions given its attributes, but the C-14 is the main sniper weapon platform.
Just some rifle trivia (shot made with a C-15):
World record
A Canadian Joint Task Force 2 sniper made the longest recorded sniper kill in history with this weapon in Iraq. He made the kill within the 30-day period leading up to 22 June 2017. The Canadian sniper killed an ISIL fighter from 3,540 m (3,871 yd).[4] The previous record of 2,475 m (2,707 yd) was set by British sniper Craig Harrison in 2009.[4]