to carry your rifle on your back, but then get it right back in your hand when you want it? Or am I missing a vanilla command?

how about just clicking to pull out your weapon?

The default key for that is "R" - if you press and hold it for a bit, you'll holster your weapon. Tap it again and it's in your hand.

eh? This sounds like a strange question because it is so obvious to do.

Once you`ve selected your weapon he will usually carry it on his back. Press the fire button and he automatically brings it out. There`s also a button to reholster. Look in your config for the key.

I do that...but I can't reholster my assault rifle when I press R again. Is there any command or mod that allows this to occur?

You have to hold down R for a few seconds to reholster it. I did see a mod that reduced the time, but I don't have a link.