I'm not huge on the whole BOB philosophy, but a lot of that has to do with my location. If a gun is to be a large part of your BOB plan and you live in an urban area, then your weapon has to be as discrete as possible. One thing that Katrina taught us was that your rights will go out the door re: the police, and you will be stripped of anything 'dangerous' and herded in with the rest of 'em. Best choice is to remain neutral in appearance and stay out of sight.
A long arm will need to be capable of being stowed in a pack. I chose an SAR3 (like an AK in .223) with a wire folding stock, because it fits in my pack, is dead reliable, won't penetrate as much in an urban environment, and is simple to operate in case someone else in my party has to use it.
A good pistol is hard to beat, though, because of the inherent concealability and speed with which it can be produced. I always have my 1911 on me, city or woods or otherwise, so that is always covered.
The ubiquitous 12 gauge is so common for a reason, too. It just works, all the time. No worries about reliability, no concerns with 'stopping power'. Ammo selection lets you work around having to use 'low penetration' ammo, or something heavier for a bigger task. It excels at small game hunting if your woods bound and works like a charm on two legged problems. The ability to get a pistol grip or folding stock for pack-ability is a plus.
Thus my SAR3, 1911 and 12 ga are my 3 'go to' guns for long woods treks, etc.
I'm not personally a fan of pistol caliber carbines because they will often penetrate more than proper .223 round in an urban environment, and do not offer the same versatility. If you're stuck on it, something like the Keltec folder is a compact little package.