A 1:7 twist isn’t as necessary to stabilizing longer bullets in a 20" barrel as it is with a 14.5". 20" barrels attain greater velocity and the bullet will exit the muzzle at a higher RPM. It’s the RPM that stabilizes the bullet, not twist rate.
There are clear advantages to the 20" barrel. One, the AR system is very reliable. Bolt velocities, operating pressures and spring rates all work in concert. The system will work with shorter barrels but it runs closer the ragged edge with a narrow margin between function and failure and require more maintenance. They are less forgiving when dirty or of ammunition changes.
As velocity is lost due to shorter barrel lengths, ammunition selection becomes more critical for terminal performance, particularly at longer ranges.
AR Carbines excel at close combat. They should, that’s what they’re designed for. But they are specialized and do not perform as well at longer ranges.
In choosing a good all around AR, the 20" is more versatile. Better reliability and durability, less intensive maintenance and a more forgiving configuration, it has it’s appeal. Though the carbine is a better choice if I need to confront a goblin in my hallway, a 20"rifle will work.
I don’t suggest that anyone should replace their AR carbines with AR rifles. But I think the 20" has lost popularity more due to fashion than utility.
The configuration I’m thinking of building is a 20" barrel with a PRI Gen III handguard. For insulation against heat, carbon fiber is better than aluminum. It’d be easiest just to use a LaRue Stealth upper. I’d like to try the VLTOR A5 stock and if that doesn’t become available, the MagPul UBR. For general purpose use, an ACOG should work. For longer ranges, a good variable.
For a carbine, I think I’ll start with a 14.5 pinned upper. I figure if I’m gonna go short, I’m gonna go short.
I’ll tell ya though- it’s sure a lot of fuss for the difference in a piece of pipe the distance between my forefinger and thumb outstretched!
