Loose rounds, yes. No prohibition on concealing ammunition. Rounds in a speedloader are also fine to conceal.
Loaded mag, no. Bad case law from the 70's says a loaded magazine
cannot be concealed if you have the matching firearm in your immediate possession.
A little clarification on the magazine issue.
If you have an exposed firearm, you can't have a concealed magazine with ammo in it as it is concidered an integral part of the firearm and may be used to make the gun ready to fire. This is considered a violation of PC 12025.
This is from People v. Hale.
Since rounds by themselves are not considered part of the firearm, they can be carried concealed with no problem. They can even be in a speed loader and not be considered part of the firearm.