I used a 1/9 twist .17 caliber liner from Brownells. The M261 barrel/chamber adapter is drilled out using a barrel liner drill to make sure the hole is centered all the way through it. The chamber is cut with a Manson .17 Mach2 reamer made for Redman barrel liners. The liner is crowned using an 11 degree crowning tool form Brownells. The liner is epoxied into the M261 barrel and then the extractor groove and spring shell holder groove are cut with a dremel.
The is the removable barrel liner setup with the drilled out GI barreled upper and the M261 unit.
This is a close-up on the M261 barrel with the liner installed next to the M261 unit.
The exact same thing can be done with a 22 caliber liner and another M261 barrel/chamber adapter. You can just slide out the .17 Mach2 liner setup and slide in the 22lr liner setup and shoot really cheap ammo! I have a 22lr version but it's cut shorter to work in a "M4 like" carbine setup I have.
I originally did the exact same thing with the Ciener conversion unit but the Ciener requires a little extra work to allow it to function correctly with the .17 Mach2 round.
This is a picture of the carbine with the Ciener that shoots the .17 Mach2 -- the barrel liner in this carbine is 17.5" long even though the outer barrel measures only a hair over 16".