Custom Activity Names in Models

I’m trying to use various custom animations inside a model, I declare it as “ACT_SHOTGUN_INSERT” however Garry’s Mod only seems to understand the stock activity tags such as “ACT_VM_RELOAD”.
I don’t get why Garry’s Mod can’t just use these custom names/tags and if it can what am I missing.
Do I have to declare the new Activity name in the QC code or is it more to do with the Lua side of things?