There are restrictions on exporting certain rifle parts, and reloading accessories, and which need export permits by US authorities,
besides the obvious, there is restriction even on magazines(clips) and certain scopes,
and some countries blacklisted as well.
I have had small parts and some accessories sent to me from the US, to New Zealand,
and a stock sent from Numrich without problems, via USPS, and declared as 1 Stock (nothing 'gun mentioned on the box)
but I think there is a value limit of $100, which is the value declared (and is actually what I paid for it plus handling etc.)
Don't know about Australia, but we have no restriction on most parts and accessories Imports here into NZ.
cheers,
SS