I have found that drilling out the top fraction of a chanter’s “throat” with a 3/16″ drill bit will flatten sharp high Gs. It will also flatten high A and sharpen F#. So far, it seems important to not drill all the way through the throat as then high G will be way too flat and the F# will become susceptible to collapsing all the way down to F (natural). Leaving 1/4″ to 1/2″ of original bore at the bottom of the chanter throat seems to give the best result.
Most chanter bores that I’ve measured are slightly larger than 5/32″ (0.15625″ = 3.96875 mm) but slightly smaller than 11/64″ (0.171875″ = 4.365625 mm), so my guess is 4.2 mm or thereabouts. Exceptions include the Ayrfire chanter at a full 11/64″ or slightly larger (4.5 mm?), as is an old David Glen chanter I have. The chanter throat is the cylindrical section of the bore between the reed seat and the main conical section where the finger holes are. The chanter throat’s length varies; I’ve measured them from 7/8″ to 1.5″ long.
If one accidentally drills all the way, or just too far, through the chanter throat at 3/16″, you can cut a short length (1/4″ – 1/2″) of 3/16″ OD (outer diameter) K&S Engineering hobby brass or aluminum* (my preference because it is softer and easier to work with) tubing and push it down into the bottom of the chanter throat to reduce the ID (inner diameter) back to “normal” as the wall thickness of their tubing is 0.014″ which puts the ID of the tubing at 3/16″ – 2 x 0.014″ = 0.1595″ = 4.0513 mm.
I’ve have drilled at 3/16″ into a modern chanter (drilled all the way through and then added tubing) and an old Sinclair (didn’t drill all the way through, left 1/4″ original bore at the bottom of the throat), in that chronological order. You can return the chanter to “normal” by inserting a piece of tubing that is the full length of the chanter throat.
Packs of drill bits often come with 5/32″ and 3/16″ bits. You’ll likely have to buy an 11/64″ individually, but it should be available at most hardware stores. However, I found the 11/64″ didn’t change the Sinclair’s throat bore enough to affect the desired flattening, but your results may vary. I did drill out, all the way, a Gibson chanter to 11/64″ with seemingly no ill effects, but also not much high G flattening either. Barely any material was removed as it was one of those bores between 5/32″ and 11/64″. If your chanter has a true 5/32″ bore to start, 11/64″ may be a good place to begin before jumping all the way up to 3/16″. However, 11/64″ OD tubing is not (as readily) available (not made by K&S Engineering anyway) so you couldn’t experiment with added tubing until you jump up to 3/16″.
Here is the video of me experimenting on the Sinclair chanter, trying to see if what I observed when experimenting on my modern chanter also helped fix an old chanter’s VERY sharp high G. It’s not a demonstration of how to do it, it is literally me filming the entire *experiment*, starting with an 11/64″ bit and then going to 3/16″ when that didn’t accomplish much. It’s 55 minutes long. Try this at your own risk.