Exactly - blanks are readily available and can be bought by anyone in the UK don't suppose it's different in the US. Live rounds on the set suggests that someone was arsing about.The phrase “loading blanks” is ambiguous, and could refer to the processes of either:
a) Manufacturing blank ammo, or;
b) Inserting blank ammo into a firearm.
I’d hope that blanks for film sets wouldn’t be made by removing bullets from “live” cartridges, as there’d be too much room for mistakes; blanks in a huge range of calibres are readily available.
Manufacturing blanks is typically done by cutting down an appropriate longer cartridge case (.45ACP pistol blanks can be made from .30-06 rifle cases, for example), charging with powder under a styrofoam retaining wad, then crimping the case mouth. The question is: why would a film set armourer bother when professionally-made rounds are available off the shelf?