National Parks = zero firearms.

If you are ever going to be in core Grizzly country (like the Teton Wilderness or other remote wilderness area in Wyoming, Idaho, Montana) and feel that a firearm is an absolute necessity (a Griz can sometimes keep coming even after a blast of spray "reportedly"), you are going to need something more substantial than a regular handgun like a 9mm, 40SW, or 45ACP. You would just piss a Grizz off if you put some nines into it. Even a 45ACP would just glance off the skull of a big Grizz.

To insure yourself against a hyper-aggressive Grizz, you are going to need either a hand cannon (like a 44 Mag and bigger) or a small rifle that can deliver a punch. For the hand cannon, they make a variety of holsters that literally attach to the front of your chest using straps that wrap around your torso. It is the most easily accessed firearm option you have with a large backpack on. But you're talking about 5-8 pounds of gun, holster, and ammo attached to your chest. I've seen hikers in Colorado with these setups and I think they are ridiculous.

If you are really serious about it, I would suggest the rifle option, more specifically an assault rifle setup. A short barreled rifle with a folding stock is compact and easily and quickly brought into service in a second's notice. I have personally BP'd dozens of miles with an AK slung to my shoulder (for fun, for shooting - nothing serious as far as defense).

The AK-47 and its dozens of variants make for many choices. The AK isn't exactly a Grizz stopper with one round, but 30 rounds of 30 caliber would be enough to persuade a Grizz to beat it. I personally have a Krinkov with a folding side stock that is under two feet in length with the stock folded. Not me, but my exact model: http://www.youtube.com/watch?v=joEzmt9kghA You get an idea of the compactness and power delivered in such a small package. And for rifle standards, it is very lightweight. To reduce weight further you can get 20 round mags instead of the standard 30's and 40's.

You do need special permits for these Krinkovs but they make tons of "AK pistols." There are many semi-auto versions on the market, and if you're a gun guy/girl, you probably already know of them. They are basically a Krinkov with no stock at all. The semi-auto versions are legal to attach large capacity mags to them as long as there is no butt or folding stock attached (no special permits needed). Here is one for $350: http://www.atlanticfirearms.com/storeproduct866.aspx Slung around your shoulder, these are very LW for a "rifle," and will deliver 7.62x39 as fast as you can squeeze the trigger.

They also make very similar products for the AR series of rifles, but I would not trust the 22 caliber round to scare off a Grizz they way I would with a 30 caliber.

A short-barreled shotgun firing slugs would probably be enough to deter a Grizz, and these can be fairly compact and "lightweight."

FWIW, I am doing a 9 day trip into the Thorofare of the Teton Wilderness to fish the Yellowstone Lake Cutthroat spawn in June/July, and this is arguably the most Grizz-infested area of the lower 48 and I am not taking a firearm and/or spray. When people tote along firearms into the backcountry for "defense," it gives them the illusion of a license to run a dirty camp. Keep a clean camp, be aware of your surroundings, give all bears a wide berth, and you'll be fine.

Also, I'm not 100% on this but even if you were to shoot a Grizz in self defense and the gaming warden was to find out, you're in for stiff fines and penalties for poaching.