Quoted:
What's wrong with a set of numbered drill bits?
We're not talking rocket science here. I would see what
fits, then go up the next size. It's going to be trial and error.
Even if you go wild and really over do it the worst can happen
is you'll be into an adjustable gas block.
Jack...
Nothing wrong with using a numbered drill bit set from a size #50 to a size #31 which would give you a
good range for AR gasport sizes that would be a "poor mans" way to get a rough/overall estimate on
your gasport diameter etc.
Most AR gasport diameters run from a low of .063" to a high of.121", Including those from SBR barrels
and those from non .223 caliber bores like .243, .257, .264 , .277 , .284 , .308 ,.338 , .451 , .500 etc.
It is of course also dependent on gas port location.
A really good adjustable gasblock like Syrac is like $110, even a basic JP version is over $60.
You can get a ballpark figure by using a quality digital caliper, while not the most accurate way
to measure your gasport diameter it will give you a basic understanding on your starting point.
Good Luck.