Like Michael said, it really depends on WHAT you're planning for. But really, my first question is: why don't you ALREADY have a pistol and extra mag(s) on you?
If for some reason the rifle go down aawww the pistol back up...... Just a thought......
...dont forget about the knife back up, followed by the last and final means of resistance, harsh language and the finger.
Ok, so here's my "expanded version" of why you should have a pistol. 1. you should already have it 2. if your rifle goes down you still have a firearm3. if you need to go "covert" "grey-man" you can hide the rifle (in a bush or your backpack - the point is, it will not be easily accessible) and still have a firearm ready4. if you have the pistol concealed, you might (theoretically) get disarmed and get lucky to still have the handgun5. if you need to do certain types of E&E, you're gonna loose the rifle and pack (think river), but you'll still have your pistol!
100 rounds of .223 in 20 and 30 round AR mags weighs 3.63 pounds .
From "Book of two Guns"-- a pistol used for fighting your way to a rifle.......