2 1/2" x 1/2" Carriage bolt, Grade 8, yellow zinc.
Carriage bolts work well in plates with 1/2" square holes. The bolt head presents a low profile to reduce splatter back to the shooter.
Grade 8 bolts are HIGH TENSILE strength steel. These bolts will resist forces that pop the heads off regular bolts. They are NOT hardened steel and can take damage from high velocity hits. Most bolt failures are not the result of direct hits but low tensile strength bolts that pop the heads off when subjected to high stress.
Grade 8 Carriage Bolts are hard to find at local hardware stores so be sure to have a few extra in the range bag.