I am on firefox and have tried dragging the ammo on the firearm or the firearm on the ammo. Neither work, it can be equipped or on the ground, in items or equipment page. I have even had the ammo and firearm superimposed in my hands. Not sure what I am doing wrong or if this is a bug.
Like earlier said, you may be using an incompatible powercell. It's all dependant on the gun/blaster you're trying to load. For example, the DL-44 was brought up earlier. It's max capacity ammo wise is a 10 round powercell. Yes, you can still load a powercell with more ammo into it, but the rest of your leftover shots will be wasted.
And thinking on this, I must question something. Are you trying to load a weapon strictly for the sake of maximizing space? Because at the current moment, combat is not implemented, so therefore will not matter if do load it successfully.
After I got firefox, then all i did was drag and from inventory to the weapon in my hand, it worked just fine. Did it twice with two weapons, both only hold ten rounds but I used a twenty round powercell and it worked. Just lost ten rounds well, twenty, but who's counting?