Just a clue for retaining "bits" on a round barrel or shaft. The firmer the basic fit, the better, line the assembly up accurately first, then apply a few drops of Loctite 390. This stuff is called "Super Wick IN" and runs like water, vanishing rapidly into every join and crevice. (Having CLEAN surfaces should go without saying). Leave it overnight while the green stuff does its thing. Next day, set the barrel up firmly held in a vise and over on its side, "BIG" side of the tapered hole uppermost. "Find" the axis of the pin holes and carefully run a FLAT ended slot drill co-axially into the holes in the gas block. Drive down until a mostly circular surface is visible. Use a stub drill or a small (tiny?) center drill to "spot" for the next drill bit. The original holes are TAPERED, so go easy and be observant when removing old ones. Once the basic holes are drilled through and are slightly undersized for the "small" end of the hole, dig out your tapered reamer kit. The tapered pins come in various lengths; get some that are a couple of inches long and that will hand-seat in the tapered hole with an inch or so hanging out both sides. Drive them in with some enthusiasm and trim and dress appropriately. Some folks use nested roll pins as substitutes for tapered pins, but I have never tried this on a gas system. Just a thought on headspacing and gauges: Headspace gauges with full-diameter heads can be a real pain at time, especially when running them through systems with spring-loaded plunger ejectors. A LOT of "mil-Spec gauges have a reduced diameter ground for the last 1/4 inch or so on the gauge. This means that you are NOT getting false readings from a dubious ejector and that small, expensive bits of measuring gear do not get spat across the workshop when you open the action. Rimmed or rimless, it is a good idea to remove the extractor and, if present, any plunger ejectors before gauging. This also provides an opportunity to find any dirt, corrosion or damage among these parts.
Not exactly sure what a 'good' brand is, I just know the stuff at harbor freight should be avoided. I spent an hour once trying to get a cheap broken bit out of an AK front sight block.
