Long-range target shooters, and some long-range varminters, go for the longer barrels solely for velocity. With any given bullet and its BC range, velocity at the far end of the flight is determined by the initial (muzzle) velocity.
Also, wind deflection at any given range and set of wind conditions is determined by the combination of MV and bullet BC. So a high BC bullet at high MV moves less in the wind which means fewer dropped points taking that shooter's wind reading skills into account - nobody reads a tricky wind perfectly for every shot, even national team members.
Whether all this matters depends on the discipline, cartridge and range. Palma, Fullbore (Target Rifle as we call it in the UK and British Commonwealth), and F/TR are limited to .308 Winchester and .223 Remington. 0.308" 155gn bullets need around 3,000 fps MV to be sure of staying supersonic at 1,000 yards, so a typical rifle for these disciplines has a 30" barrel minimum, sometimes up to 34", to get the necessary MV.
The downside of length is weight, as all of these disciplines have weight limits. F/TR which has the highest at just over 18lbs including bipod sees the shooters have to choose between barrel length and profile, scope model etc to stay within the limit. Accuracy comes from a heavy profile barrel, but add that to 30" or more length and it's very heavy - that's why you have the 'Palma' barrel profiles that are heavy at the breech end for about 6", then have a steep taper down to a smaller diameter.
You're talking about .223 Rem where barrel length has less effect on velocity than in larger cased cartridges. There is very little velocity gain by going above 30", and you don't lose much by dropping to 28".
If you're not shooting at very long ranges, say at 600yd max, you really don't need a super-long barrel. 26" is probably all you need, and go for a heavy profile instead which makes it stiffer, hence likely to be more accurate.
Incidentally look at some pics of the short-range benchrest guns in the main site. Since they are used at 300 yards tops, and there are weight limit issues, you'll see they are built with short (20" or even less) but very fat barrels - accuracy beats velocity in this game.
Laurie,
York, England