Well here it is over 4 years later and Remington still hasn't made a threaded barrel for the R51. I was about to make my own barrel with I discovered the R51 chat room and there was a guy that made some threaded R51 barrels. Here's the link to the company: https://www.dangerco.us/
They are $125. Bought one for my gun last May. It took me a little bit of tinkering to get my Thompson Machine BDSD (a variant of their Poseidon wipe can) to screw on. There was some tolerance stacking going on. The Dangerco barrel was at the high end of acceptable thread pitch dimeter and the TM BDSD was at the low end, but with a little work and the judicious use of a 1/2-28 tap and die I got it to fit.
This is an awesome combination. The TM BDSD, like the Poseidon is a wipe can and is meant to be shot wet. Charged up with water based wire pulling gel and a fresh wipe, I can get about 10ish rounds through the can before I need to change the wipe and recharge the gel. To my ears, it's on par with my Sig P228 with my Octane 45 (with 9mm end cap) quiet for the first 4 rounds. All in all it's everything I hoped it would be when I bought the gun and the can. It sucks that I had to wait so long to enjoy it.
Pictures to follow.