Always use a either a 308 Carbine/Rifle spring on blowback AR9s.
These blowbacks rely on a strong spring to keep the bolt in battery. If the bolt "unlocks" too early, it'll cause a number of issues. You want the bolt the stay forward for as long as possible to keep the gasses moving forward behind the bullet. The longer the bolt is able to stay in the "closed" position, the less dot movement you will see. Also, the violent cycling of AR9s reek havoc on trigger pins. Slower, the better. A lot top GM shooters that run AR9s tend to run heavy bolts as well. Some even use a steel weight plug between the buffer and bolt in the buffer tube, as to slow down the cyclic rate. I noticed around .02-.03 slower splits with a weight. However, it's totally worth the trade off when the dot hardly moves.
Hope this helps ya'll