It's probably current Post Office rules; when I was working we couldn't post Lithium batteries by themselves, however if they were in a camera it wasn't a problem. Please don't ask because I'm still struggling with that one myself!
A friend in the north east owns a 25 Pdr Field gun and does the reloading himself using a cut down .303 case as a primary ignition source; he wanted some more empty cases so I posted off 96 "brass tubes" as it was the only way to get round the Stasi who were running the place.