This is the Back portion of the Fuse Barrel System by Lapco (with Desert Edge).  The bore size of this back is .691, and is threaded for Auto Cocker thread paintball guns.
FUSE Barrel Features:
8 inch control bore
Reverse threaded tip so that you can unscrew the whole barrel without also loosening the tip. Useful for pump players where the pump handle makes removing barrel backs difficult.
FUSE backs sized in .680, .683, .687, .691
Compatible with Planet Eclipse Shaft 4 backs and tips
Eclipse backs available in .681, .685, .689, .693
Combine with a full eclipse kit for a total of 7 backs
14 inches long with tip
Five rows of progressive two-step porting.
Quiet, and cleans out very well.
Minimal excess material for a light weight, but strong barrel that is thin enough to not interfere with pump handles.
Barrel fluting reduces weight, helps porting clean out better, and is a useful grip surface for removing the barrel.
Attractive hourglass shape at middle joint, and at the back.