Just finished a new SPR MK12 Mod 1 build. When I test fired it today, during the first few rounds, only the first round would go off, then the second round wont even get chambered, then the trigger wouldnt reset. Then after a few magazines, the first round would go off, and then I get a double feed. The lower is a used Aero Precision with just milspec parts, and the upper receiver is a Cerro Forge. Im currently running a Colt Carbine buffer spring and a Colt H1 buffer in a SOPMOD stock. I also have access to a Springco blue spring and an H2vbuffer, butvI didnt try using it yet. Any idea about whats wrong with the rifle?

Will the bolt lock open on an empty mag?

https://www.ar15.com/forums/AR-15/Troubleshooting/66/

Sounds to me like it's way undergassed.  Sometimes GB has to seal up with carbon from a few rounds, but really shouldn't be doing this.

GB centered properly over port?

What brand barrel? Gas block properly aligned? Gas key bolts aren't loose/broke?

Likely undergassed as mentioned. 18" rifle gas barrels aren't ideal. Need a port at least 0.098. Some manufacturers go bigger.

I've read in an AR15/M4 armorer's manual that 9 rounds must be fired to ensure a proper seal.