Where do I get Concussion Rounds?

Concussion Rounds is a random drop from The Circuit in Duviri and from Aurax enemies, as the drop tables list.

Naramon

Concussion Rounds – Mod Stats per Rank & Drop Locations · noxsframe.com

Duviri/Endless: Tier 1 (Normal)	Legendary	0.67 %
Duviri/Endless: Tier 3 (Normal)	Legendary	0.67 %

Scrofa Heavy Trencher	Rare	0.44 %
Scrofa Legionary	Rare	0.44 %
Scrofa Light Trencher	Rare	0.44 %
Scrofa Provisor	Rare	0.44 %
Syzygy Light Trencher	Rare	0.44 %
Taro Ranger Crewman	Rare	0.44 %
Terra Cestra Manker	Rare	0.44 %
Terra Elite Overtaker	Rare	0.44 %
Terra Elite Provisor	Rare	0.44 %
Terra Elite Trencher	Rare	0.44 %
Terra Jailer	Rare	0.44 %
Terra Manker	Rare	0.44 %
Terra Nullifier Crewman	Rare	0.44 %
Terra Overtaker	Rare	0.44 %
Terra Provisor	Rare	0.44 %
Terra Trencher	Rare	0.44 %
Vapos Detron Ranger	Rare	0.44 %
Vapos Elite Ranger	Rare	0.44 %
Vapos Nullifier Ranger	Rare	0.44 %
Vapos Sniper Ranger	Rare	0.44 %
Vapos Tech Ranger	Rare	0.44 %
Vorac Ranger Crewman	Rare	0.44 %
Deimos Runner	Uncommon	0.33 %
Orphid Husks	Uncommon	0.33 %
Volatile Runner	Uncommon	0.33 %
Aurax Actinic	Rare	0.17 %
Aurax Vertec	Rare	0.17 %
Axio Detron Crewman	Rare	0.17 %
Axio Numon	Rare	0.17 %
Axio Vambac	Rare	0.17 %
Axio Zerca	Rare	0.17 %
Detron Crewman	Rare	0.17 %
Narmer Crewman	Rare	0.17 %
Narmer Detron Crewman	Rare	0.17 %
Orm Detron Crewman	Rare	0.17 %
Orm Numon	Rare	0.17 %
Orm Vambac	Rare	0.17 %
Orm Zerca	Rare	0.17 %
Taro Detron Crewman	Rare	0.17 %
Taro Numon	Rare	0.17 %
Taro Vambac	Rare	0.17 %
Taro Zerca	Rare	0.17 %
Vapos Detron Crewman	Rare	0.17 %
Vorac Detron Crewman	Rare	0.17 %
Vorac Numon	Rare	0.17 %
Vorac Vambac	Rare	0.17 %
Vorac Zerca	Rare	0.17 %
Orb Vallis - Enrichment Labs Enemies	Rare	0.14 %