Just picked up a Ciener conversion.  Thinking about converting this to a dedicated upper eventually.  Does anyone know if the CMMG collar is compatible w/ the Ciener kit for this purpose.

that is what I used to build mine with works great

I believe the CMMG is a Ciener copy. The copyright expired a long time ago, Arthur W. Ciener .22 kits have been around since the 1960's.

I have a dedicated .22 upper that uses the Ciener kit minus the chamber insert. The bolt face mates to the rear of the barrel upon closing.

I still own the chamber insert should I ever want to try the kit in a standard rifle. Accuracy Speaks built the upper for me.

Yep, mine works great.

I have a ciener kit from the early 80's and it has a different firing pin that dented a chamber on a dedicated upper I had. The firing pin is wider. I haven't looked at it for a few years so I can't be a lot more specific.

The CMMG kit is essentially the same as the .22 LR AR-15 conversion that J. A. Ciener still sells.  The inventor of the kit was Max Atchisson.  He invented the assembly in 1970 when he was an employee of Military Armament Corporation (M.A.C.) who initially sold the units in the early 1970's.  Ciener bought the manufacturing rights in 1993.  A more complete history can be found on pages 192-194 of the book about Gordon Ingram and the guns he invented, "The MAC Man" by Frank Iannnamico and Don Thomas.  The conversion kit patents would have expired some time before the Clinton Assault Rifle and High Magazine Capacity Mag Ban expired in 2004.

MHO, YMMV, etc.