Assuming the guns are target trained, it is much more easy to store a pile of ammunition somewhere and tell everyone to come and get some in an emergency, than having to transfer the rifles whenever someone decides to move. The alternative of course is no personal ownership of the rifles, but aside from the familiarity and training it also adds a symbolic sense of responsibility and association. The scene in jarhead comes to my mind where they are told to make this “there is many like this, but this one is mine” chant over their marksman rifles.