I just had a similar experience but on 7.62x39
There are 2 things you can try before drilling out your gas port:
1) knock out the pin of the buffer and remove the tungsten weight, which is about 1.4oz
2) if that doesn't work you can try to cut the coils one at a time of the help spring in th buffer, I had to cut 5 coils to get my system to work.
Or you can start drill the gas port, measure your existing and drill in small increments
Here's some pics to show you what I'm talking about, below is the sbr in question, though it's a little unique because it's a piston:
To make it work reliability I tried in order:
Take out tungsten weight
Drill out gas port from .07 to .091 to .125
Add a krink device.
All of this still did not work on 7.5" barrel,
Then I switched had to:
switch to 8" barrel and drilled out the port to .125
Cut the buffer helper spring to the minimum length
Then it was holding then bolt back properly. After I added the tungsten weight back in and it worked properly
Hope this helps
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