I paper-tuned 2 different arrow setups, one being what I used last year – 340 spine with 12g insert and 100-grain field point, and new for this year a 300 spine with 100g insert and 100-grain field point. Arrows at 29″ and bow set at 70lb. While shooting at 20yds, everything seems fine and groups well, but I backed up to 30 and there was noticeable erratic arrow flight from both setups. Accurate and hitting the right spot, but not pretty. I’m lost.

While there could be a couple different possible tuning issues, I believe there is one main issue causing this. You are most likely under-spined, even with the 300 spine arrows. For your specs, the 340 spine arrows are right at the cut-off and are possibly too weak. As for the 300's, those are definitely too weak with a 100-grain insert, and 100-grain field point. I would recommend replacing the 100-grain inserts on your 300 spine arrows, with the 12-grain inserts, and see if that fixes your flight issues.