Originally Posted by WoodenPlank
Most 18" barrels are drilled for rifle length gas systems, but I'm pretty sure I have seen midlength before, too. Either should serve you well.
Twist will be determined by what types of ammo you want to shoot. If you just plan to shoot cheap 55-62 grain ammo, then 1/9 is fine, but will limit how heavy you can go. 1/8 will allow for heavier bullets (up to ~80 grain), and 1/7 will allow for that and some of the extra long projectiles (M856 tacer, >80 grain). Some folks have been unsatisfied with the accuracy of 55 grain ammo from 1/7 barrels, but it's never been an issue for me.
Also, bear in mind that there are no absolutes - every barrel is different. Some 1/9 barrels will hold nice groups with 75-77 grain ammo, and some will group more like a load of buckshot.
In my opinion, a 1/8 twist is ideal for almost any application in an AR, short of a dedicated light varming gun slinging 40-50 grain bullets. It'll handle ~50-80 grain projectiles, 70 grain Barnes TSX, and should shoot most loads with acceptable to exceptional accuracy.
So, perhaps I should look for an 18" with 1/8" twist.
Where do you stand on the question of stainless vs not, and heavy vs not.
Again, keeping in mind that this mainly a range gun. Either my backyard range, and some varmits, or gunclub ranges and showing off my gun's accuracy
Probably not competitions, but maybe down the road if I think I'm doing well and have time on my hands, and if I want to learn more about rifle shooting.
Hone defense use too, but not buying it specifically for that purpose