Wouldn't be surprised about the gas port as it's supposed to be very small in diameter (smaller than the M1A and Garand), and apparently any sort of tampering will cut off the gas entirely.
It's also worth noting that this barrel's about 19 inches long by default, but I believe the tri-compensator (7" version) should even that out somewhat.
well, all things being equal, the larger the gas port, the lower the pressure at the gas port
and the larger the volume of gas that escapes the barrel through the port in a given time interval. while the system is pressurized, those hot gasses must have just enough pressure to drive the action to unlock, extract, eject, and cock the weapon while the pressure in the chamber is low enough to safely do so.
if the
correct amount of gas reaches the action too quickly (before pressure in the chamber is lowered to a safe level), the action might try to unlock and extract early, leading to case rim tear, failure to extract (where the extractor slips off the case rim), and increased wear on locking surfaces.
if
too much gas reaches the action (where you have the gas system pressurized too long, the work needed to be performed is not enough to use all the power from the expanding gas, an excessive amount of gas in the system), then you will see evidence of cratering on the primer as the hot gasses "flow" around the material of the firing pin / striker and partially into the firing pin channel. you will also feel excessive recoil, encounter increased wear on locking surfaces, damage to pistons, springs, bolts, carriers, et c. you will also have similar problems mentioned with the action opening too quickly: torn case rims, head separation, early unlocking, failure to extract, et c.
a combination of both issues usually means that the gas port is physically too large or is misplaced - large enough to allow too much gas, or is placed too close too close to the chamber, which keeps the system pressurized longer than it should be.
your primers show evidence of flow, and the case rim tears are indicative of a gas pressure problem. you also indicate that the first half dozen shots were fine. this would suggest that either there is a problem where the gas port or gas cylinder's size has increased (somehow), or you have something that is messing with the pressure that the system expects (debris, very hot ammunition, parts not assembled properly).
given that a drilling fixture is often used for these sorts of rifles i would put money on the gas port being oversized. a wobbly drill bit, a crooked fixture, et c can do this.
