I picked up a Colt HBAR 20" upper this week with the 1-7 twist.   This is my first of this kind.  My other three AR's have a 1-9 twist.  
I am hoping to be able to shoot heavy grain bullets a bit more accurately with this new set-up.  
Are there any advantages to the 1-7 outside of the 68-77 grain bullet weight?  
How do these perform with light bullets such as the 55 grains or  even smaller?
Also, what is the cheapest quality heavy grain commercial ammo available now?  I have a few boxes of Hornady 75 grain TAP on hand, but it's a bit expensive for target plinking.
It seems the 1-9 twist is the most popular, I hope I am not going to be dissapointed with my 1-7 Colt!

I love my Bushy 1/7.

With 77g Sierra MK, Ive gone out to 300yds but will go farther pretty soon after fall harvest when I get my long ranges back.

Heavier bullets is the biggest benefit of the faster twist rate. You don't really want to shoot anything lighter than 55gr. though.