Key Holing

What is Keyholing?

Keyholing occurs when a bullet tumbles in flight and strikes the target sideways rather than point-first, leaving an elongated hole that resembles a keyhole. This bullet tumble happens when the projectile lacks proper stabilization, typically due to a mismatch between your barrel twist rate and bullet weight.

When a bullet keyholes, accuracy becomes impossible. Instead of consistent groups, you’ll see erratic impacts with bullets hitting the target at various angles. In extreme cases, rounds may miss the target entirely as the tumbling bullet loses its intended flight path.

Why Keyholing Matters for AR-15 Builders

Understanding keyholing helps you select the right barrel for your intended ammunition. The most common culprit is using heavy grain bullets in barrels with slower twist rates. For example, a 1:9 twist barrel may stabilize 55 grain projectiles perfectly but cause 77 grain bullets to keyhole at longer distances.

Common scenarios that cause keyholing:

• Wrong twist rate: 1:9 or slower twists with bullets over 69 grains
• Damaged crown: Even microscopic damage affects bullet stability
• Undersized bore: Manufacturing defects that squeeze the bullet
• Low velocity: Subsonic loads may not spin fast enough to stabilize

Preventing Keyholing in Your Build

When configuring your build on the AR15 Outfitters builder, match your barrel twist to your intended ammo. A 1:7 twist handles everything from 55 grain to 77 grain bullets reliably. For primarily lighter bullets like 55 grain FMJ, a 1:8 twist offers the best balance.

If you experience keyholing with factory ammo, first verify your twist rate matches the bullet weight. Handloaders should check that bullets aren’t seated too deep or crimped excessively. Remember: proper stabilization depends on bullet length more than weight, which is why polymer-tipped bullets often require faster twist rates than FMJ rounds of the same weight.