I need to pin the A2 flash hider on a 14.7 inch barrel. What size drill bit and pin to use?  thanks.

I've read 1/8" works well. But I've also read that people have used piano wire and coat hangers as pins so..... I sent my 14.5" out to have the muzzle device pinned and from the size of the weld I'd say an 1/8" pin of some sort was used.

I usually drill the hole and then cut the back of the bit off and use that for the pin. About a 1/8" or silghtly bigger is right.

I did the same thing as well. This way the blind pin when fitted will not have a loose fit. Also using the shank of the drill bit, being a high carbon steel; you take advantage of the steel's strength. In my case, I used a 3/32" drill bit; but if I do another blind-pin; I will use an 1/8" drill bit.

On a side note; as for the filler wire I used to weld over the blind-pin, I chose a silicone bronze filler wire because of not knowing the exact grades of steel between the flash hider and the blind-pin (ref. welding of dissimilar steels).

Proper welder for the job is a TIG, but I use a small Mig because that's what I have. Just have to make sure to cover everything except what I'm welding because of splatter. Alumnium foil works for this.

I use 3/32 dowel pin. I grind it to about 1/16 longer than the hole is deep and then TIG the pin to the hider. No filler materiel required.

Piano wire is fine, but I definitely would not use a coat hanger.  Very soft steel, would easily shear and may not even damage the threads.

As a rule of thumb, I'd say anything you can easily bend back and forth to failure with your bare hands from a 2" length or effortlessly snip with a pair of $3 side cutters is not sufficiently strong.

Just remember that whatever it is, it needs to render your flash hider non-removable with common, conventional means (wrench, channel locks, vise grips, etc).  If ATF were ever to test it, they will put 200 ft-lbs to it, and it has to either take it or destroy something.  If they're able to break it loose, but the barrel threads are destroyed in the process, it was permanent.  If they snap the muzzle device, it was permanent.  If the barrel twists or spins in the receiver, it was permanent.  But if it comes off with no or minimal damage to anything (i.e., you could screw it back on or easily clean up the threads with a thread chaser), then it was NOT   permanent.

Obviously having to drill it out, split the muzzle device or take an acetylene torch to it is above and beyond common means of removal with hand tools.

I drill and tap the FS and drill a good deep dimple into the barrel and use a 4-40 set screw then tig weld over.

This