Both regular version of M1 and M2 carbines have glitched ejection port (the bullet can be seen through the ejection port) while the character pulls the charging handle. However, both carbines' Rifle grenade version are fine. It's a minor visual glitch, but I just want to point out the inconsistency. Please see the video linked below.
https://youtu.be/lLisG_eUZVg
(The first and the second are grenade version of M2 and M1 carbines, then the last two are normal version of carbines, you can clear see the ejection port on regular version is glitched while chambering a round)