Optimal Barrel Time
Also: OBT
A load-tuning theory that times the bullet's muzzle exit to a favorable point in the barrel's vibration cycle to tighten groups.
Optimal Barrel Time, usually shortened to OBT, is a theory proposed by Chris Long that ties accuracy to when the bullet leaves the muzzle relative to a shock wave traveling up and down the bore. The idea is that exit at certain calculated time windows coincides with a steady moment in the barrel harmonics, so shots placed there should string less vertically. A handloader picks a muzzle velocity that lands inside one of those windows and works the charge weight toward it.
OBT remains a debated model rather than settled science, and many shooters reach similar charge weights through ordinary load development without ever running the math. It is best understood as one framework among several for predicting an accuracy node, useful as a starting point but not a guarantee. Treat its predictions as a hypothesis to confirm at the target, not a replacement for live-fire testing.