The Altra Insert and Outsert.166 350 Spine is a modular tuning accessory for archery arrows, designed for shooters who want precise spine alignment and weight control to achieve consistent, accurate flights.

• Diameter:.166" insert/outsert compatibility for easy integration with standard arrow systems
• Spine options: Available in 250, 300, 350, and 400 spine to match shaft stiffness and shooting style
• Weight: 50 gn total weight to help balance your arrow and optimize center of gravity
• Package: Sold in a 12-pack for convenient replacements and multiple setup configurations
• Compatibility: Accepts micro FMS insert weights for precise, incremental tuning

This setup enables you to fine-tune arrow spine and overall weight without replacing entire shafts, delivering steadier flight and tighter groupings. Use it to tailor arrow behavior to your draw length, target distance, and preferred point-of-impact, whether you’re dialing in for target shooting, field rounds, or backyard practice. The combination of selectable spine sizes and micro-weight options makes on-range adjustments quick and repeatable, reducing guesswork and helping you reach your consistent best.