The .416's would be a little pricey to just go plinking with for me. $1+ a pop just for the bullet.
I haven't found a tapered expander for this. I was going to turn one on the lathe, but just don't have the time right now. I think that RCBS all uses the same thread so if one was made it might be able to be used between the 444 and 44mag die which ever one would work the best. I was going to get a 444 die, but Midway is out.
I have a metal .223 single stack on the way to see if it might be useable. In an AR mag the round just noses into the front of the mag and the last round just pops right out. Maybe someday there will be an AK 450BM magazine.
For the .375, can the std 7.62x39 be necked up? No fuss with mags and bolt face opening.