Looking to assemble a decent rifle for "1-2" groups at 100 yards. Not looking for a super tack driver, but if I can beat 1: groups at 100 yards with a 6x scope and an off-the-shelf upper. Should I look at a 20" barrel? Would a 24" inch barrel w/ 1/8 twist give me even better performance? with a 6x scope?

Just curious what you might do.

20" is plenty for that. You can go shorter if you want.

Thanks cttb, I have a gaggle of shorter carbines, nice and fun, but I'm looking for an excuse for something more varmint-ish I guess. Would 20-24 get me better performace than 1" at 100 yards with better optics in teh future? I'm thinking base, off the shelf parts and components, nothing real fancy, just pressing the AR platform to its best performance without breaking the bank. I figure I ought to be able to do pretty well.

Thanks for the reply.

A 20” barrel is long enough for any commercial ammo I am aware of.
If a person is going to reload to take advantage of a longer barrel it can be worthwhile.
At a hundred yards barrel length and scope power are not factors. YMMV

You won't see any benefit at all at 100 using a longer barrel than 20"  You'll get a little bit more velocity, but you won't get any more accuracy.  I find AR's longer than 20" to be a bit on the awkward side.

20" it is then. I've had no trouble hitting a pie plate at 100 with iron sights, even keeping a pretty decent group, just hoping to tighten it up a bit. Thanks for all the advice. Glad I asked.

Barrel length has nothing to do with accuracy. I have 14.5's that will shoot a half MOA. For what you want, just get a decent 16" middy.

Agreed.  Any good quality 14.5, 16, 18, 20, or 24" barrel will be fine at 100y.  You really only need the velocity of a longer barrel to beat wind and push your transsonic range for 600+ yards.   Groups at 100 are more about fundamentals, match ammo, and barrel quality.

5.56 was designed for a 20 in. barrel....

Also buy a book called The Marksmanship Primer. Read it cover to cover. The most important spec sheet you can find on a rifle is that of the person holding it.

A 24" barrel is wasted on shooting paper at 100 yards.   Accuracy at that distance is a function of barrel quality and not length.  And, no, the two do not necessarily go together.   If you want sub MOA groups at benchrest distances, then get a match grade 20" barrel, typically stainless, with a twist rate that is optimal for the weight and length of bullet you will be shooting.  Think Kreiger or Lilja as good examples.

Now, having said that, if you ever start to use the rifle for shooting beyond 100 yards, especially for hunting beyond 200 yards, the extra velocity from hand loaded 5.56 bullets and slow burning powder can give you up to 200 fps more velocity and that translates into more retained energy for a clean kill, a flatter trajectory and for really long shooting, an extension of the distance as which the bullet goes transonic and possibly destabilize adversely affecting accuracy.  That is not likely until you approach perhaps 600 yards, however.

MS556, great info, and everyone else as well. I could see going out to 300-400 yards with optics but I'm not there yet, I'll go with a decent 20" barrel and see how far out I can get with it satisfactorily. The wealth of info and book references are much appreciated.