I want to switch out my 1x9 16" Barrel on my AR with a 20", but I can't decide between the 1x9 twist rate or the 1x7 twist rate. I plan on shooting M193 and M855. So what are the advantages/disadvantages, what would you get?

Quoted: I want to switch out my 1x9 16" Barrel on my AR with a 20", but I can't decide between the 1x9 twist rate or the 1x7 twist rate. I plan on shooting M193 and M855. So what are the advantages/disadvantages, what would you get?

The 1:9 is more versatile and the 1:7 allows you to shoot up to at least 77grns.  Purely depends on the type shooting and what you prefer.  If you plan on getting in any firefights at distances much over 100 yds I recommend 1:7 (Or if you're a distance shooter of any type.)

I don't know what type barrel you're after nor your use but Wes Grant (mstn.biz) has one very sweet, and high-priced, 18" Douglas.  (By 1963 and the AR-18 ArmaLite had proven you could get 3000+ fps out of an 18.25" barrel.)  

For general all around shooting the 1/9 is probably the best bet.  It is exactly how 5sub describes it.  In the Marine Corps, we use the 1/7 used to stablize the Tracer Rounds we fire.  I could be wrong but the 1/9 is what I have on the majority of my rifles.  I do have a Colt HBAR 1/7.  You might pay a little more for the barrel.

I hope this helps.

Max

Quoted: For general all around shooting the 1/9 is probably the best bet.   I could be wrong but the 1/9 is what I have on the majority of my rifles. I hope this helps. Max

Yep

1:9 is the most versatile and the most common twist and that twist is what we're most likely to have.  (I still love the old preban AR-180's and those have a 1:12 twist barrel - perfect for properly (accurately) stabilizing 55grn Mil-Surp.)