The Altra Insert and Outsert.166 400 Spine is a precision arrow component designed for archers who customize their shafts, offering flexible spine options and fine-grain weight tuning to improve consistent flight.

• Pack Quantity: 12 pieces per package for reliable tuning and spare replacements.
• Spine Options: Available in four spine sizes — 250, 300, 350, and 400 — to match your arrow shaft stiffness and flight characteristics.
• Diameter:.166 inches, designed for standard insert/outsert systems to ensure a precise, concentric fit.
• Total Weight: 50 grains per insert-outsert assembly for predictable weight distribution when tuned with micro weights.
• Micro FMS Insert Weight Compatibility: Accepts micro FMS insert weights for fine-grain balance adjustments and point-of-impact control.
• Design: Integrated insert and outsert construction provides secure attachment and repeatable alignment on compatible arrows.

Using this setup, you can tailor spine and weight distribution to your shooting style, reducing flight variability caused by subtle changes in balance. The 12-pack enables quick experimentation with different spine sizes and weight combinations, making it practical for builders, seasonal archers, and those adjusting for varying distances. With micro FMS weight compatibility, you can dial in precise impact points to compensate for wind, elevation, and torque, delivering reliable accuracy across practice and competition sessions.