I found this MAC Atchisson device today in my travels. I gave $75.00 for it. No instructions.

I wonder if CMMG or Black Dog mags would work with it?

Any magazine that works with other Atchisson-derived conversion bolts will work with it.

Black Dog makes magazines for many different conversion types, but their magazines for Ciener/CMMG types will work.  CMMG mags will work.  Original Ciener mags will work, etc.

I'd say you got it at a good price.

I have one and a few mags.  

The mags almost work with my Cieners, Spikes and CMMGs. The .22 mag itself is the same as the metal Cieners but the outside appears to be a M16 20 rounder with a floorplate that is marked for the Atchisson unit. Springs on a couple of the mags are weak and I think the feed angle is slightly different.  One day when I get up the energy - I'll replace the springs and see if they work.

The unit itself will not fit in any of my uppers - military and commercial - so I don't know if the BDMs, Ciener and CMMG mags work.

The instructions are fairly simple. I have two slightly different copies:

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YMMV