take a used or new cartridge, remove the primer if one is in it.( i use 243 or 308)
i wont insult anyones intelligence re safety with ammunition.
once you have removed the primer, place the shell case in a drill vice and drill a 8mm hole 2 cm from the mouth of the case.
then clean all burring and any loose metal shavings from this hole. it must be nice and clean to enable you to get any sound. i use a needle file to file the underside of the hole.
take a 3cm length of 8mm dowel. as for the sprig whistle shave the top of the dowel off so the remains looks like a semi circle of wood.
insert into the shell and push so that the end of the dowel is just visible in the hole you have drilled or is just behind it, you will need to play with it to get the best sound.
then glue this in place.
once set trim, polish and blow.
this photo just gived you an idea of the shape of the dowel.
and here are two i made today, both 308 and same hole etc but the dowel is in slightly different in each.
and even better they come with a ready made groove for a lanyard.