You actually don't. I posted a link that shows velocity vs. barrel length, and the expected velocity gain of that extra 1.5" barrel length will be anywhere from 50-100 fps. For paper punching (like the OP is wanting), that is a negligible difference.
I've seen several other tests that say that a 24" barrel makes you loose velocity. There are just too many variables when it comes to these tests. You would need several guns with the same brand barrel made with the same tools to really test it. Most .223 loads are designed for a 20" barrel.
I much prefer a 16" barrel or longer. From my shooting I have noticed a significant difference of accuracy between 14.5" and 16" barrels. As I stated in my post above you can't really tell the difference until you get out to longer ranges. I have found that a 24" barrel is more of a compromise than an improvement. I actually get better results out of a 20" barrel than a 24". The extra 4 inches adds weight up front, and decreases the maneuverability and versatility of the gun.