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. .125" is the max gas port for the AR-15. Most all gas blocks have a .125" hole to align over the port. So it'd be impossible to get perfect over the gas port.
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.125" is the max ID of the hole for most gas tubes and that is the correct. But most gas blocks have ports larger than that. Consider that most set screw gas blocks use a 10-32 set screw. The drill bit used for a 10-32 tap is listed as being a #21 bit (.159") or 5/32 (.156"). So many mfr's use the same bit for drilling the port as well as for tapping the set screw hole.
Regarding alignment, the Govnah has an alignment port to get perfectly over the port in the barrel as seen in the picture below.
Once you have the shank of a drill bit or gauge pin aligning everything, then you tighten the set screws.
We don't have an alignment port on the baby Govnah, so you can see that we countersink the restricted port by using the same 10-32 tap drill to allow for some misalignment.
If you just want a set it and forget it setup then the baby Govnah is the way to go. Just drill out the restricted port to your desired size and you're done.
Regarding .088" note that there is no drill bit that is nominally .088".
See this link:
http://bobmay.astronomy.net/misc/drillchart.htmThat chart covers fractional, metric and numbered sized bits. There is a .086" and a .089". There is no .088" drill bit.....I understand that you may have a .088" port as may be determined by a gauge pin set and that is possible since bits can drill under and over sized holes but I personally think it makes more sense to reference actual drill bit sizes that are readily available.