This might be fruitless, but I'll take a shot.
I recently added a Riflespeed Adjustable gas block to a 7.62 UU. Barrel is the 16 inch PredatAR with mid length gas system. The kit came with, what I would guess, a Sprinco buffer spring (I say this due to the red on the end of the spring) and an H2 buffer. When I first shot the rifle, I used Malaysian surplus L2A2 ammo. The ammo was purchased from Sportsmans Guide a few years ago when the Malaysian Surplus was available (sometime during the Trump years). The only issue I had was light primer strikes. As far as I can recall, I did not have any problems with last round bolt hold open.
After adding the adjustable gas block, I could not get a setting that would provide a consistent last round bolt hold open--even on the highest setting (most gas), the bolt did not consistently lock back. I contacted Riflespeed, and they were helpful. Was sent a shorter plunger for the adj. gas block. But, I still had inconsistencies with the last round bolt hold open. The plunger they sent almost completely allowed for unrestricted gas at the highest setting. Yet, I had a few instances of the bolt not locking back on the last round.
I bought the block because I have a suppressor in waiting (in case anyone is curious), and because I liked the concept of a tooless adjustable gas block. Now, I did not try different ammo yet. Nor have I tried a different magazine (I was using a fairly unused Magpul ten round mag).
I disassembled the gas system. The gas tube is not restricted by anything (I ran a bike cable through the tube to see if anything was blocking it). The gas port on the gas block is in the same position as the port on the Larue gas block. The barrel was dimpled, so there should not have been any issue of misalignment. The gas port on the barrel is on the small side (~.0625"). But, I don't know why this would cause inconsistencies. Unfortunately, I did not take a photo of where the brass was landing. It was approximately around 5 o clock between 4 and 6 feet away. The brass did eject, the bolt just wouldn't always lock open on last round--regardless of setting.
Below is a photo of my gas port. The drill bit is a 1/16" bit, and it fit snug in the port.
Attached File Attached FileThis is a photo of the gas block on setting 12. This is looking through the screw hole to the gas port. Arrow is pointing at the end of the plunger (which you can barely make out).
Attached File