This is probably the most important question to ask. You will never get a full gas impulse with a barrel that short. I built a 9" .300 BO SBR that functions flawlessly with a JP Silent Capture buffer and spring assembly. Start switching between different buffer weights. It's the least invasive of the variables to improve cycling reliability.Some more info is needed. Did you build from a stripped upper or was it purchased as a complete upper. If the former, what barrel and gasblock was used? Have you tried blowing compressed air down the gas tube to see if its obstructed? Which buffer weight are you using? Which bcg?