50 Beowulf is a nice, big, fun sledgehammer of a round developed by Alexander Arms. Although it's available in other platforms, its main claims to fame are that it's a 50 caliber round that can be fired from an unmodified AR-15, using unmodified magazines (for best results though, use the right follower), and (most importantly) is usually fired from relatively inexpensive AR-15 upper receivers. For half the price of a Desert Eagle pistol, one can have an Alexander Arms upper for an AR-15, firing the same projectiles but with a larger cartridge. Ballistically speaking, it's similar to a 12ga slug at 200 yards, and after that the projectile drops like a rock. However, hunters report the Beowulf as being incredibly effective on moose and other large game, so it seems to be doing something right. On a side note, Magnum Research's Performance Center offers the BFR in 50 Beowulf. However, Performance Center revolvers aren't cheap, and I believe the Beowulf BFR starts at around $1500.
The Beowulf cartridge is probably the best "let's do something weird with the AR platform" round for a pre-equipped reloader to get into. Although the brass itself is a special order item, it's the only unusual part about the Beowulf setup. The cartridge's base is identical to that of a 7.62x39mm round, so there's no need for custom shellholders. The cartridge's diameter and projectiles are interchangeable with that of the 50 Action Express, as long as the dies are threaded far enough to accomodate the longer length of the Beowulf cartridge (or conversely, the shorter length of the 50AE cartridge). If you happen to own an SKS or AK-47 and a Desert Eagle, you either already have, or will eventually wind up getting, everything you need in order to reload 50 Beowulf anyway!
A Note To Californians
There are some misperceptions out there about the Beowulf and California law. California's recently enacted 50 BMG ban only affects the 50 BMG cartridge, not other 50 caliber cartridges. The Beowulf is not affected by this ban. The caliber is absolutely California-legal; any rifle which would be legal with a 223 upper will remain just as legal with a Beowulf upper.
Any 7.62x39mm shellplate/shellholder will do. I deprime using a standard Lee shellholder and a hand press, and load using my Dillon 650 with the 7.62x39mm shellplate.
Dillon 650 Specifics
You can't use the case feeder OR case tube with this caliber. You have to remove all the insets and such from the feeding system on your press, and drop in cartridges by hand. While you COULD insert them at the bottom of a stroke, in practice the cases will frequently catch and not drop into position to be pushed onto the shellplate. I've found the best results come from dropping the cases in when the press is at the top of its motion, and I haven't had a case get stuck once with this method.
The Lee die set, which I have, includes a correct size expander to bell the mouth of the brass. While I intend to install a Lee powder measure as my next experiment (got one cheap), right now I'm using the Dillon powder measure to drop powder into the cases. I actually have put the Lee expander in the position immediately after the powder check, where the seating die would normally go, and have installed the seating/crimping die in the last station. While unorthodox, and resulting in a different reach than I would normally have, this does get all the necessary operations done to load a round.