If you look at e.g. this
http://www.ar15.com/archive/topic.html?b=6&f=21&t=327387
A little bit down there's a cutaway and diagram of a 44mm canister round.
As I understand it, all 40mm rounds have a complex high/low explosive
arrangement that is required to not punch holes in the shoulders of the
person firing the round.  Also even buckshot rounds use a custom sabot
rather than a simple cup like with shotgun buckshot.  This is the reason I
didn't make 40mm rounds craftable when they were added.
Basically once you get above a certain size, you have to go to great
lengths to keep it manageable.  I could see a reloaded round that has much
lower performance, but crafting milspec 40mm rounds seems excessive.