I have a DPMS panther bull 20. It is very heavy gun. A 24in or 26in would be even heavier.
Bull barrel - stationary shooting, bench shooting, can shoot 10 and 20 shot groups with little deviation.
HBAR - more mobile, very accurate, see Colt HBAR, RRA coyote, Bushmaster Predator
standard barrel - plinking and CQB, 1.5MOA groups
pencil barrel - plinking and CQB, 1.5MOA groups, cheaper than standard barrel