Here's a little how-to for you. If you've ever had a gas block that is held on by set screws come loose, you know what a pain that is. I saw it happen to a guy at a 3 gun match this year, and it ruined his stage and his match score. I'd recommend this to anyone using such a gas block.
Since I'm not competing any more this year, I decided to rebuild my upper. I would start by replacing the previous GB Hunter gas block with a micro gas block and pin it. I'll use a standard taper pin used in a standard front sight base. It shouldn't go anywhere.

Step 1: Setup the mill
I dimpled the barrel where the set screws go to hold it in place during the rest of the process. I'll also use the screws in final assembly for redundancy. Here you see my setup in the mill. This method of clamping works just fine for drilling gas ports, but caused me problems in later steps.

(My phone takes a decent picture!)

Step 2: Establish coordinates and spot face
I started off by touching off on the barrel OD and calculating the bore centerline. I established this as zero. I pinned the gas block .375" from the center. This would barely skim the barrel but it would keep me in the "meat" of the gas block and provide plenty of contact with the pin. Anything as close as .300 away from the bore center line will work.
To keep my drill from leading off, I took a .125" carbide end mill and made a spot face just deep enough that the tip of the drill would land on the machined surface.


Step 3: Drill (.125")
Here's where I ran into trouble. The drill I first used was HSS and dull as Al Gore, so when I tried to start the hole I turned the barrel slightly. I then ditched the v-block and clamped the gas block directly and switched to a carbide drill. No problem after that.


Step 4: Taper Reamer
We're getting close. This is the important part. I clamped the barrel in a standard vise and chucked the 2/0 taper reamer in a hand drill. Go slow and check your pin depth often. I drilled about 1/16" at a time. Take the assembly out of the vise and use a bench block or something similar, put the pin in the gas block, and beat the crap out of it with a brass hammer.


If the pin isn't approximately centered, repeat the process.


Step 5 (Optional): Grind pin
Since the micro gas block has such a taper, quite a bit of the pin sticks out of either side. I'll mark it tomorrow and grind it to be a little shorter.


Then it's down hill from there. I'll pull it apart and cerakote both parts. I marred some of the existing barrel cerakote in the vise. I'll then nest it inside a carbon fiber handguard and reattach the compensator.
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