ok, guys, help me choose my first AR. I haven't decided which manufacturer I'm gonna pick but I'm leaning towards Rock River Arms. What I can't decide on is barrel length. I know it depends on what it will be used for but what is the maximum effective range of the 16-incher as opposed to the 20? I wish I could have both but the fund won't permit that right now.

if you're going to hunt or target shoot, get the 20"

everything else, 16".

that is MHO.

What is your definition of "maximum effective range"?  Your idea may be different than mine.  The US military says the M4 with 14.5" barrel has a maximum effective range of over 300m.

Thanks for the opinions and info.

I guess what I should have asked is what would be the difference in group size at, say, 300 yards. I would probably never have to make a shot any longer than that. If the accuracy difference isn't that significant then I will probably go with the 16.

Barrel length has nothing to do with accuracy.  In fact short barrels flex less so they are MORE accurate at shorter ranges.  The bullets have lowered velocity and the exteranl ballistics will require more of a hold over at long range however.  (You wont need to hold over until past 250+ yds with a 16" barrel) You also lose terminal performance faster with a shorter barrel.  Expect fragmentation to 100yds and accurate fire to 300yds with the 16" barrel and M193 ammunition.

I selected Colt.

Purchased a 20" Match Target Comp HBar, equipped it with a 9 x 28 scope. Then bought a Colt 16" barrel M4, soon to receive an Aimpoint for a sight system.

Either way you are on the right track for a well built, reliable, fun weapon.

As for selecting a manufacturer: you'll get lots of opinions here at AR15.com. Listen to them all, then head out to your dealer and examine the rifles and pick the one you feel suites your budget/needs best. Good luck.

The AR-15 Ammo FAQ at www.ammo-oracle.com has some excellent information on maximum effective range for M193 & M855 from the different length barrels.

For target shooting with iron sights, the 20" is theoretically more accurate than the 16" because of the longer sight radius.  With optics it doesn't matter.

If you're going to seriously target shoot, or shoot at long ranges (300+ yards) then I'd suggest the 20" barrel.  For everything else, I'd probably get a 16" barrel.  But try out a few of each if you can, and take the one that that feels best.  (For example, IMHO the short handguards on the 16" are too short so I'd want the mid-length or 20" instead. YMMV)

As far as manufacturer, figure out which configuration and features you want, then look for that.  Bushmaster is probably the "best" for a general-purpose AR based on features like chrome-lined barrels and 5.56mm chambers being standard.  Armalite and RRA are almost as good, with RRA the best value.  DPMS & Oly are OK too.