As to who make them, HW and Walther Lothar both sell choked barrel blanks.

As to why, erm let us know , I believe the notion is it reforms the pellet on exit reducing pellet fussiness, I may be wrong on that as I have never heard a definite statement as to the reason.

All I know is after testing a choked barrel for grouping then cutting the choke off and re crowning, the groups did not change one bit, so now I only buy unchoked barrels from Silco as they are cheaper.