Firearm Convention used a barrel with an initial 25-inch length. After firing a total of 5 rounds, they cut off 1 inch. This was repeated until they were left with a 7-inch barrel. The list below shows the average velocity of the 5-round shoot for each barrel length. Your results will vary based on individual rifle, ambient conditions, and the specific factory cartridge you choose.

The trend suggests there isn't much ballistic advantage beyond an 18 inch barrel. A 35 fps difference of three foot instrumental velocity is meaningless at 200 yards and may be more than compensated for by using a higher ballistic coefficient projectile. It is strike velocity that matters, not muzzle velocity, unless you plan to shoot deer off of your muzzle. 

On of the best (at least on paper) is the Federal Fusion Rifle 350 Legend Part # F350LFS1. It is a 160 grain bullet at 2300 fps, with a .259 G1 static BC. Federal says 1600 fps is the minimum strike velocity for terminal performance. I presume they mean reasonable bullet deformation, often called “expansion.”

It is hard to call the 350 Legend a wonderfully impressive load, as it isn't. After 215 yards, strike velocity drops down close to an ethical minimum level and wind drift becomes an issue as well. G & S Online Members can refer to Chuck's expanded rifle ballistics summary. https://www.chuckhawks.com/subscribers/tables_charts_lists_page/rifle_ballistics_summary_exp.htm