and you will be needing to purchase the gun to get it...
That would be the Breech/barrel assembly off a Cap-Chur .50 Tranquilizer Dart gun which were based off of the old Crosman 160 and was designed to run off CO2.
Cap-Chur has been around for years. The Breech & barrel is a one piece unit with a nicely rifled .50 bore. There is no loading port, you pull the entire bolt from the rifle to load the Tranquilizer Dart so to do the conversion will require some small mods, mainly to cut a loading port and shorten the barrel.
These were designed to mount on a Crossman diameter gas tube so the way to go would be a 2250 length pressure tube set up for the Disco valve and fill, cut for the Cap-Chur strikers side cocker and drilled and threaded for the action screws and a 22XX grip frame with the barrel bobbed back to around 12"
You can find these off & on if you hit Gun Broker or Auction Arms and will usually run in the 300 dollar range but you do end up with your breech, barrel and a better striker set up and end cap.
The other route would be to go and use the Cap-Chur bolt assembly and fashion your own breech around it off of 1/2" bore DOM steel tubing then use a .50 Track of the Wolf barrel liner. Really wouldnt take much to build a far nicer loooking breech with simple tools
To those who may scoff at the idea you should be reminded that this was how DAQ got into Air Guns. Back in the early to mid 80s Dennis sold Tranquilizer Dart guns, initially of the Cap-Chur line then later his own clones again built on Crosman platforms. Many of the basic features of a DAQ Big Bore were directly taken from Cap-Chur Dart guns, particularly the side cocker striker. Dennis refined the design a great deal but the roots are still visible.
Cap-Chur is still around today so you may be able to just buy the bolt and striker assembly from them
heres a link:
the other point is one could use much the same components to piece up a decent .50 rifle using a aftermarket QB stock and trigger group with a bit of work
food fer thought guys!