I read some where, I think it was here, about how you shake the bullets to see if they rattle to make sure they are blanks, have I made this up or is that really a thing?
I think that's only a very specific type of blank.
I think that might be for rounds that look real but have zero charge, not even the percussion cap.
Watching a video by an armourer the other day (from several years back) and he's talking about all sorts of different "blanks" including "blanks" that have something like twice the gunpowder of a normal round and thus the gun has to be modified to take the extra pressure.
In theory dummy rounds rather than blanks, rounds which look identical to live rounds but are inert, are supposed to have beads inside so you can tell by shaking them when used in this kind of context - I don't believe there is any standard for it though just a best practise thing.
Yup
The video I was watching was saying how different armourers can have very different ways of making blank firing guns depending on the type of gun (so what is mechanically required), the budget (do you make multiple ones for what might be only slightly different uses, or one that can have parts swapped), personal preference and what is needed to be shown on camera.
One example he gave was one method to make a gun a "blank" firer had the potential to throw back burning powder towards the person holding it, another iirc increased the pressure coming out the end of the barrel so you had the choice of the person holding it potentially getting stuff in their face or a slightly more dangerous level of pressure at the front. The result was you might want to have two guns even with the same "blank" rounds depending on the shot.
He was also saying he went about making them slightly differently to many others because he got into it from IIRC a career as a machinist rather than a gunsmith so used different methods/tooling and looked at them from a different direction.
Basically there doesn't appear to be any standard way because there are so many different guns that might need only slightly different modifications, or completely different ones and different people approach things differently depending on training, viewpoint, available equipment, what the shot is going to be, budget and speed even on the same guns (like many things).