My Enfield carbines are 18.7" barrels. This is the standard No5 Mk1 length. FAL's 18" even. CZ 527 7.62x39 carbine 18.5".
That's a very good length for a carbine, it's still short enough to manage on mobile operations and it doesn't sacrifice too much velocity. A 1:10 twist barrel will provide a very good range of ammo one could utilize with this length.
My savage bolt is 20", AR10 is 18". Both have their specific roles and do them well. My old 700 had a 24", not anymore accurate than the 20". I think 20" may be the sweet spot.
Understanding twist rate to length ratio and ammo is paramount. If you are your barrel, then you're doing it wrong. If you can fire your weapon accurately and hit your target, then it doesn't matter what velocity you've got, a kill shot is still a kill shot. Don't make excuses and don't expect to have every advantage, because all the equipment in the world cannot take the place of skill.
My 24” Stainless Bull Barrel mounted on my DMPS Custom AR10, was chosen for it’s Long Range capabilities, and not for it’s easy humping around the woods… It turns out to be a Good Gong Ringer out at 1000yds, when firing a custom Barrel Specific Loading…
I tested a Spanish FR8 with and without the flash hider/grenade launcher at night using Portuguese surplus 7.62x51 ammo. (The flash hider resembles an M16 bird cage. Just slightly larger) With it, I could barely make out an occasional stray spark. Without it, the 17.7" barrel made an impressive fireball. I can break 5" clay pigeons at 100 meters, off-hand with this handy little rifle. Good enough for me.
