Certain models seem to get stuck on distant resolution LODs when first loaded into the game. Once instantiated, the model will remain locked on that res LOD, and will not change even after switching to a new mission. The only thing that fixes it is to relaunch the game and hope that the model is not bugged when you load it again. It happens very rarely, but affects only a select few weapons very consistently. It happens with default Arma 3 (no mods loaded) and with mods as well.
I've also been told that the Vermin SMG used to suffer from this issue and was fixed, but I've never encountered it myself.