At about $25 each, are the HK barrel flute brush, chamber face brush, and receiver/mag brush worth it? Recommended? Has someone found cheaper or better brushes?
If was only getting one, it would be the chamber face brush. On 223 one can make do with a USGI chamber brush and use the steel brush part for the locking lugs to do some of the cleaning, however, the "HK" face brush being larger still works better.
If you are using a larger diameter pistol brush to clean the chamber, make sure it does not have cut steel wire ends of the center support that may damage something and a brass core / support wire would be best (factory chamber brush has a front section that fits the bore).
For either the face brush or chamber brush you need a short (medium) length rod that does not turn, a M16 handle and rod section or two (second for a handle) will work, may need an adapter for threads.
Have run across other round brushes that fit the receiver and mag, over the last thirty years ;-)