They shall not be more than 34 inches in length; nor more than 2 5/8 inches in diameter, and if wood, not less than fifteen-sixteenths (15/16) inches in diameter (7/8 inch for bats less than 30”) at its smallest part. All composite barrel bats shall meet the Batted Ball Coefficient of Restitution (BBCOR) performance standard, and such bats shall be so labeled with a silkscreen or other permanent certification mark. The certification mark shall be rectangular, a minimum of a half-inch on each side and located on the barrel of the bat in any contrasting color.

2-1/4″ metal/alloy barrel – allowed with BPF 1.15 stamp
2-1/4″ wood – allowed
2-1/4″ composite barrel – NOT allowed
2-5/8″ metal/alloy barrel – allowed with BPF 1.15 stamp
2-5/8″ wood – allowed
2-5/8″ composite barrel – allowed – MUST be BBCOR
2-3/4″ not allowed in any Little League Division regardless of material

2-1/4″ and 2-5/8″ “Hybrid” or “Half and Half” bats which are composite handles with metal/allow barrel are allowed with the BPF stamp.

Quite simply, if there is composite in the BARREL, in must be have the BBCOR stamp.