I have a Geisel super gas block and a ba 11.5 barrel. The gas block is tight against shoulder but the port hole is 75% open.  Not sure which is out of spec but what’s a work around to reposition ?  I could possibly sand .010 off gas block or take some meat off barrel shoulder but don’t have a lathe  ….    Any suggestions?   Thanks

I'd say the best course of action is to try another gas block and determine which part is the issue. Then, contact them about the problem.

Shim it off of the shoulder or source another GB.

Misread the OP.

I'd determine where the spec is off personally.  I'm guessing the journal OAL or barrel port location.

Thanks. Will check dimensions — my description may no va been worded correctly

Your description is fine. I may be overdue for a few days off work.

The distance between the journal and the port is spec'd.  What is the dimension you get from the journal to the center of the hole?

Stupid question but I have to ask.  Are you sure your alignment is off in the right direction?  You say it is 75% open with the GB up against the barrel shoulder.  A brain fart could easily have your problem bass-ackwards and it needs to be backed off the barrel shoulder to align it properly.  I have seen people slam the GB up against the shoulder several times and cause a partial blockage of the gas port, but it is really rare, especially with decent makers, where the gas block or gas port was mis-drilled or the barrel shoulder is in the wrong place.
If you are correct though, and another gas block does not solve the issue and you don't want to send the barrel back for replacement, yes, you can use a belt sander and take off some off the face to get it to align but I would quadruple check to make sure the grinding off the face of the gas block will solve the issue without creating another, such as setting back the gas tube too far and causing it to bottom out in the gas key, which might happen if the gas port or shoulder is indeed in the wrong location.  My money is on a brain fart that has you thinking backwards and moving it off the shoulder will correct it.  I've been wrong before, in fact, several times just today, so good luck.

And some are designed to mount tight to the journal step, others take into consideration ~0.020" for a handguard retainer.

If it comes down to hacking on things (not saying it should), I'd just enlarge to hole in the gas block to accommodate the misalignment. It won't change the location of the gas block (and as was mentioned the gas tube) like sanding down the side of the block will.

No  not a stupid question,,, it’s. Installed correctly but won’t line up….  Will borrow another another block and good point about the tube length. Thanks gentlemen

True, but this guy is far from that number.  Also, gas block orifices are twice the size of typical gas ports so that difference means nothing to gas flow.

Yeah, something is fuckey with this.

FWIW, the G SGB that I have here has a gas opening that measures 0.0123" in diameter, for reference.

Using a borescope to see if it's obstructed, somehow?

They are indeed all over the map, but always larger than the port in my experience, to allow for any slight innacuracies in alignment.  I've never owned a G tho, and that is pretty tight.

I always turn the GB 180 degrees and see where the port lines up with the first set screw hole, as that is also where it will align with the orifice.  The only GB's I have had lately that were offset for the handguard cap have been fixed FSB's

Just following up.  The gp was located .243 off the shoulder ,, removed .004 form gb,,,, just cleared.  I t was close.   Thanks for ideas.   Cheers