Bank 50 motes every Gambit Prime match for a chance at the curated roll. There is also a chance that a random roll drops at the end of a match. Your best shot is Tier 3 Reckoning when the Shade of Oryx is the boss. On an easy modifier day like Attrition/Grenadier. A smaller loot pool than Gambit Prime but still rare. I have only had 1 of each weapon drop and tons of the Outlast Pulse Rifle. Never a Spare Rations from Reckoning.