You can swap barrels, but it's not really a "switchbarrel" rifle. You can, however, swap out upper assemblies in about 2 minutes. That's if you're slow. I've got a couple of ARs, one has a 20" heavy stainless barrel (RRA Predator Pursuit Upper) and the other one is a fairly generic 16" carbine length upper. The heavy barrel is definitely easier to shoot accurately but I haven't spent a lot of time with the shorter one as it's only got iron sights. My eyes are getting a little too old for that.
With a good barrel, trigger, and optics an AR can definitely give bolt actions a run for their money. I'm still working up loads for mine, but I've found two that are sub-1" @ 100 yards with 5-shot groups and I know several people with the same upper and barrel that can get 1/2" groups on a regular basis. For most hunting I'm going to do, that's good enough for me.
Here's the caveat though: I don't know if I'd use the same trigger for an HD gun as I have in a hunting AR. I have an RRA Varmint trigger that is right about 3.5 lbs which is lighter than I would really like for HD/SD. Adrenaline can make that feel like a couple of ounces, at least from what I've heard from people who've been in real-life situations.
It's hard to own just one. I've got 4 lowers and 3 uppers that will be getting built into something over the next couple of months. I'm a blued-steel-and-walnut (or baltic birch) kind of guy and I'm sold on ARs.
Matt