diff --git a/docs/shader-tuning.md b/docs/shader-tuning.md
index b57cad4..fb2c93e 100644
--- a/docs/shader-tuning.md
+++ b/docs/shader-tuning.md
@@ -71,3 +71,12 @@ tuples carry the 12th element (=2), so L gets BK_STEP=2 while M/S inherit defaul
 | 128 (L) | 28.1 | **28.08** | held (L win kept) |
 | 256 (L) | 28.5 | **28.46** | held |
 L-path 28 t/s WITHOUT the M regression. Shippable champion. Progression (L): 5.8→23.4→26.2→28.1.
+
+## Iteration 6 — BK_STEP=1 (full 2048 occupancy attempt) — REGRESSED (barrier crossover)
+| ubatch | iter5 | iter6 | |
+|--------|-------|-------|--|
+| 128 (L) | 28.1 | **23.1** | ↓ 4× K-loop barriers outran the occupancy gain |
+| 256 (L) | 28.5 | **23.3** | ↓ |
+BK_STEP=2 is the occupancy sweet spot; reverted to iter5 champion (28 t/s). Shared-based occupancy
+exhausted. Next lever: **f16 accumulators** (register-based occupancy, no extra barriers) to break
+the 33–64 reg band toward 2048 resident threads.