Originally Posted by ithaca_deerslayer
So an 18" will have the standard rifle gas system, just like a 20? Is that generally true, or just with some 18"?
At this point of my learning, I can envision a full length rifle setup, full handguard, full stock, fixed or detachable handle, 18" or 20" barrel.
A detachable handle and 18" barrel both make sense to me. So does a heavy stainless barrel if it is noted for accuracy.
Not sure on the twist yet.
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.