While I'm swapping barrels I was thinking about going with a free floating handguard. How much of an accuracy improvement is that usually worth?
I can't quantify that with a "minutes of angle" answer, as all rifles are different in the area of free floating. Having said that, it makes sense that removing any external forces that would press against your rifle's barrel would also remove some level of inaccuracy.
Virtually every shooter on this forum will tell you that a free floated AR barrel will shoot with fewer flyers and hold better groups. Free floating removes one more variable from your shot preparation.
If you'd like to read some examples, there is plenty of evidence on some of the boards where people have written about their experiences replacing their hand guards with free float tubes.
In my case, I've never owned an AR without a free float tube, and they all shoot sub-moa. My .223 will shoot 1/2 or 1/4 moa if I control caffeine intake, and my .264LBC shoots roughly 3/4 moa. As usual, YMMV.
My experience in the old days was I would see about 1/2-1 MOA smaller groupings with the FF tube over the standard set up. Now that was the OLD DAYS. Barrels are shooting far better now and even the regular rigs are stellar at times. My goal would be 1/2 MOA with a new one. Getting a full MOA out of one would be very difficult IMHO.
The best firearm for self defense? Any loaded one will do.