I just bought a used one of these, the complete dedicated .22 rifle version not the standalone upper. There doesn't seem to be too much info out there on these. I've already broken it down, and it doesn't look like i'll be able to install a standard buffer tube/stock on this as it's the weird-o proprietary Bushmaster Carbon 1 piece stock. Anyone have any experience swapping in a standard stock on these?

Also, in case anyone was wondering the upper and lower only fit on each other; not swappable to standard AR-15 uppers/lowers. I guess this is why they've been discontinued.

I've only messed around  with the upper mounted on a standard A-2 lower.  The upper came with its own buffer spring assembly and you had to swap buffer/springs to use the dedicated upper.  The one I played with had an integral weaver rail which reminded me of the tactical carry handle that comes on some RRA rifles.   One of the major issues with the Bushmaster dedicated polymer 22's were the unreliable magazines.  Black Dog Machine saw that the mag for the M-261 could be tweaked to work with the carbon 22 so high capacity mags are now available but weren't when the rifles were sold.  The upper I shot had a plain barrel with no flash hider or muzzle break.  The upper was pretty accurate and functioned pretty well with the 10 round factory mag.

Yup, that's essentially the description of my upper. I just gave Bushmaster a call to find out if they still make the replacement stock and/or if a standard AR buffer tube can be attached. The answer was no for both questions.