Brief update - spoke with Rainier today after a few emails back and forth. My opinion of the company remains unchanged - one of the best to work with. They confirmed my gas port size is what they were expecting it to be.
Confirmed my troubleshooting steps so far. Am going to do a few more things:
Run a pipe cleaner down the gas tube to check for obstructions.
Replace my lube (an oil/grease mix that is in fairly common use) with aeroshell only (Rainier recommendation)
Substitute a lower off another rifle to see if problem follows the lower - possible friction/binding in the buffer tube, from misalignment of the upper/buffer tube, possible trigger/hammer interference, or tensioning screw issue.
Replace gas block and gas tube. I've got another gas block on order.
If none of these works - they asked me to send the assembled upper to Rainier for them to try on one of their armory lowers and replicate the issue.
Confirmed my troubleshooting steps so far. Am going to do a few more things:
Run a pipe cleaner down the gas tube to check for obstructions.
Replace my lube (an oil/grease mix that is in fairly common use) with aeroshell only (Rainier recommendation)
Substitute a lower off another rifle to see if problem follows the lower - possible friction/binding in the buffer tube, from misalignment of the upper/buffer tube, possible trigger/hammer interference, or tensioning screw issue.
Replace gas block and gas tube. I've got another gas block on order.
If none of these works - they asked me to send the assembled upper to Rainier for them to try on one of their armory lowers and replicate the issue.