I'm working on building my mark 12 mod 0 clone and have a few questions regarding the installation of my PRI front sight block.  I aligned it to the indexing notch on the barrel opposite the gas port, but the sight seems slightly canted.  I aligned the sight properly but the index seems not quite as centered as it was but definitely still in the ballpark.   Is this really bad or might cause short stroke, explosion, etc. ?  Any advice is appreciated.

It sounds like the dimple is just a little off, but it shouldn't matter.  With the sight straight, it should be directly over the gas port hole.

The gas block's hole is bigger than the barrel's port so you have some room for error.  This picture will give you an idea of the amount of play you have.



If it helps, mark the barrel shoulder at top dead center using a straight edge between the barrel's index pin and the center of the gas port.   Also, put a mark on your sight directly below the middle of the gas tube hole.  Line up the two marks.  If the dimple is slightly off the gas block might try to shift while tightening up the screws, so watch the marks to make sure they stay aligned.

 

great photos and information.........I usually eye ball them and see what happens when I start shooting.......a little tweaking may be in order and I try to get a mechanical zero If using iron sights..........once everthing is gtg I use red loctite...........

That was awesome!   Great way to do that.   Thank you for the info and I got it perfectly on there now using that method.  I think I put a little too much faith in the index dimple.

Great explanation. . .

Glad to see it worked out for you.

Here are the rest of the pictures that describe one method for installing a set screw gas block.  If you needed to dimple the barrel I think the easiest way is to use a jig like the one sold by Rainer.