Hey,
Something I think is quite strange that I wouldn't mind hearing what others think but if you follow the description text for bolt weapons in FS and if you have looked at how bolt weapons are described in DH/RT, non-Astartes bolt weapons use a smaller calibre shell than Astartes bolt weapons (except one bolt pistol in Inquisitor's Handbook). Is it therefore more realistic to say that the Marines in FS and OE cannot replenish their ammo as they have no access to Astartes compatible bolt shells as it is unlikely a unremarkable Agri-world would just happen to have Astartes equipment just lying around?
I know I will allow my players when I run it to be able to replenish their ammo, otherwise they will have to club everything to death after a couple of fights, but its interesting to think about.