Hello
What are the Gas Port Diameter on Standard AR 5.56mm 16" and 20" barrels?
Thanks

PS this is what I found so far:
M4 (14.5" Barrel?) incl. SOCOM - 0.063"
M16A2 (20" Barrel?) - 0.093"
Mk18/CQBR (xx" Barrel) - 0.070"
Mk12 (18" Barrel) - 0.0995" (Crane-spec)???

I’ve also seen Mk12s listed at .104. I’d favor the smaller number.

14.5 and 16 carbine are .063” from Colt but I like LMTs .058”.

I found this.
http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html
Lots of variance in sizes.
What I can tell you is that the .070 gas port that came on my PSA 20" barrel did not work.

It is not so exact as only one specific size for all ammo for a particular barrel.  For instance, a 16" carbine port location, .750" journal size may run great at .063" with full power 5.56 ammo, but have trouble cycling weak bulk ammo or lower velocity match ammo.  That barrel may cycle .223 Rem match ammo best with a .078-.080" port, but seem overgassed with full power 5.56.

I'd rather be a bit overgassed with the hot stuff and be able to reliably cycle my .223 Rem match ammo.

.063 is intentionally overgassed for weak budget ammo.

You can go to a .0595“ gas port on a 16“ carbine.

This is what I have seen over the years

For 5.56mm
Colt's gas port sizes are:
* 10.5" - .093
* 11.5" - .081
* 14.5" - .063
* 16" - .063

Lots More info here

There's a number of reputable online publications listing "standard" gas port sizes for different barrel and gas system lengths.

TOS had a somewhat recent thread where a member put together a Google doc/spreadsheet; with accompanying form for anyone to contribute verified port sizes. It wasn't an incredibly active thread, but there's some decent data.

It was a cool idea. I think he even provided an option to list when the rifle being measured was purchased (to help identify any mfg changes over the years). It wasn't really intended to establish "correct" sizes. Still, it gives one a good idea of what they can expect from some of the common mfgs.

Thanks - where can I find the TOS link?

My sub MOA match ammo needs a larger port.  Lilja knows that and ports its original NSWC spec SEAL Recon match grade 16"  barrel at .080".  They got it right for the ammo It likes best.

Yes, it ejects full power 5.56 at about 2 o'clock, but my match loads back at 4 o'clock.  I run an H2 buffer.  I know ejection pattern can be unreliable, but it handles everything without a hiccup.  A smaller port would not likely cycle the  Berger/Lapua  1/2 MOA match handloads.  Ammo port pressure is a factor.  We do don't all shoot 5.56 pressure.