I'm a big fan of the .22 caliber for a number of reasons. With that in mind, I have a few of them in my collection with a few more to go (namely, a PMR-30). In the pile is a 3-screw Rugar Single Six, an older Remington single shot bolt action (to get a threaded barrel at some point), the ubiquitous 10/22, and an AR-7.
The AR-7 is a great little rifle but if I was out in the field, I probably wouldn't want to have it collapsed and stored and risk the 45 seconds it would take to assemble it. I'd have it built and strapped to the side of the pack within easy reach.
If I had my druthers though and the cash was available, I'd keep the AR-7 stored except when out actively trying to get small game (it's primary use IMHO) and keep a Single Six holstered and loaded with .22 WMR. It adds more weight to the BOB but provides a greater level of versatility. That's assuming though that we were only looking at .22 firearms.
With no restrictions except that it were to be a firearm that could reasonably be added to a BOB (i.e. no AR-15's or AK's), I'd toss the AR-7 in the bag with a full size pistol in 9mm or .45 on top.