It’s got a huge gas port, so I don’t know if that’s the issue. I ordered an adjustable gas block since it seems we might be completely over gassed.
You might be over-gassed.
My only experience with PMC Bronze were chambered in 223. I found it to be a low pressure loading with lots of velocity variation over the chrono.
If that is the case in their 308 offering, you might be running a light projectile with low pressure gas. In an over-gassed system, the round might be sufficiently under-gassed to balance everything else out.
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That said, my thinking is this:
(1) "Huge gas port" is relative, specifically to dwell time.
Your 16" rifle gas system has a large port and a short dwell. Presumably, these things are developed in conjunction through testing, to
allow larger quantities of gas to be pushed into the action in a shorter amount of time to provide adequate energy for extraction. (Could have been developed for a specialized purpose (heavy metal competition, dedicated suppressed system).)
(2) Loads are developed to stay below a certain peak pressure (for all the reasons we can't exceed certain peak pressures in a rifle) rough various means (from quantitative testing through wild-ass-guessing).
All other things being equal, lighter projectiles can have more powder behind them. Less mass = faster acceleration = increased volume earlier in the burn = lower peak pressure.
This results in a higher energy system, with more gas pressure at the port.
(3) For gassers, the net energy required by reciprocating system for positive extraction is influenced by the degree of brass contraction.
FTEs are the result of inadequate energy applied to the reciprocating system.
If extraction is started before the brass has contracted, you need more energy to overcome chamber drag.
The longer the distance between your chamber and your gas port, the more time the brass has had to contract.
If you were running a shorter gas system, chamber drag would definitely be higher on my list of likely culprits.
Running rifle gas system gives brass more time to contract of the walls of the chamber before the extraction process is started.
Running a rifle length gas system in a 16" means the dwell time (gas port to uncorking distance) is (relatively speaking) short.
On paper, these elements point to an under-energized system.
My bias is to reduce spring rate when troubleshooting under-energized systems (my original comment), as I prefer slower cyclic rate systems.
If the adjustable gas block doesn't work, maybe look to reducing spring rate or lightening the reciprocating system.
Good luck.
